**3.3 Diameter regulation**

*Nanorods and Nanocomposites*

of ZnO nanorods on the patterned area. Such a nanorod-array structure with high surface-to-volume ratio favors the adsorption and desorption of NO2 gas on the

*FESEM images of the selective grown ZnO nanorods micropattern: (a) entire pattern, (b) edges of the grown* 

The influence of different rod growth times on the morphology of ZnO nanorod films was investigated by FESEM with cross-sectional images illustrated in **Figure 8**. The ZnO nanorods become longer but the diameters of the nanorods nearly keep constant with the extension of the growth time. For example, the lengths of the ZnO nanorods grown for 2 and 4 h are 500 and 1300 nm, respectively, but their diameters are nearly the same in the range of 50–80 nm for both cases. Nanorod morphology has strong influence on the response performance of the sensor. The increasing nanorod aspect ratio increases the nanorod surface area and thus allows more gas adsorption interface. Larger fraction of the nanorod is depleted upon adsorption of NO2. Longer growth time of the ZnO nanorod sensor results in longer nanorod so that the corresponding sensor has higher sensitivity

*FESEM images of the cross section of ZnO nanorods prepared with different rod growth time (a) 2 h and* 

**42**

**Figure 8.**

*(b) 4 h.*

sensor.

**Figure 7.**

than that with shorter growth time.

*area, and (c) enlarged images of the nanorods.*

The size (diameter or length) of ZnO nanorods grown via hydrothermal method is affected by several factors such as concentration of precursors, growth temperature, and time. The average diameter of ZnO nanorods can be regulated by changing growth time. The average diameter is about 35 nm with the growth time of 0.5 h. Upon the growth time over 1 h, the average diameter rapidly increases up to 100 nm when the growth time is 2.5 h. But then the average diameter keeps nearly constant as the growth time is elongated [23]. ZnO nanorod arrays were successfully synthesized on Si substrates using two-step route including spin-coating seed on substrates and chemical bath deposition (CBD) growth. The diameter of ZnO nanorods can be controlled by changing seed density on substrates or NaOH concentration in solution. Increasing spin-coating times leads to better seed density so that the diameter of ZnO nanorods decreases from 150 to 70 nm as the spin-coating times increase from 1 to 50. And the diameter of ZnO nanorods obviously increases with NaOH concentration increasing [24].

Solvent polarity has a special effect on final dimension of the ZnO nanorods synthesized using a one-step solvothermal method. The diameter of ZnO nanorods decreased with the moderated polarity by introducing less polar ethanol solvent. The diameter of the ZnO nanorods can be regulated by adjusting ethanol content in the solvent in that the diameter of the ZnO nanorods decreases as ethanol content in the solution increases. **Figure 9** shows SEM images of samples a0, a10, a20, a30, a40, and a50 prepared by introducing 0, 10, 20, 30, 40, and 50 vol% ethanol solvent, respectively. **Figure 9(a)** shows ZnO nanorod surface full of flocs when

#### **Figure 9.**

*SEM images of ZnO nanorod samples (a) a0, (b) a10, (c) a20, (d) a30, (e) a40, and (f) a50 prepared with 0, 10, 20, 30, 40, and 50 vol% ethanol solvent.*

**Figure 10.** *Response and recovery curves toward 100 ppm ethanol at the optimum working temperature of 370°C.*

prepared in pure water solvent. The flocs disappeared upon the addition of 10 vol% ethanol, as shown in **Figure 9(b)**. When ethanol was increased to 20 vol% (**Figure 9(c)**) and 30 vol% (**Figure 9(d)**), ZnO nanorods show uniform, well-defined, and welldispersed morphologies. The average nanorod diameter decreased from 360 to 220 nm when ethanol percentage in solvent increased from 10 to 50%.

**Figure 10** shows transient response and recovery curves of the ZnO nanorod sensors toward 100 ppm alcohol vapor at the optimum working temperature of 370°C. Here the sensors fabricated using samples a0, a10, a20, a30, a40, and a50 are labeled as S0, S10, S20, S30, S40, and S50, respectively. It is apparent that S20 is much superior to others. The inset with a single response and recovery curve of S20 measured at the same condition indicates that its sensor has the response of 42 and very sharp response and recovery time of 20 and 8 s, respectively [25].

## **4. Different structures formed by ZnO nanorods**

#### **4.1 Cross-linked configuration**

Ammonia sensors based on ZnO nanorods (NRs) with a cross-linked configuration has excellent sensing performance by shrinking the interdigitated electrode spacing d. The electrode spacing d, working temperature, and gas concentration strongly influence the steady- and dynamic-responses and the related repeatability and different gaseous response performance [26]. Reducing the electrode spacing d increased the ammonia sensor response S because the configuration of ZnO NRs is transformed. The studied sensor with an electrode spacing d of 2 μm at 573 K shows a highest ammonia sensor response S of 81.6 toward 1000 ppm NH3/air gas and could detect NH3/air with a lower ammonia concentration of 10 ppm. Moreover, the response S of the ammonia sensor is temperature dependent, as is mainly attributed to reactions of oxygen species. The adsorption-time (τa) and desorption-time (τb) constants of the studied sensor (d = 2 μm) at 573 K are less than 3 min. The improvement of ammonia-sensing ability could result from the formation of more cross-linked configurations. Finally, the studied sensor with a cross-linked configurations shows good ammonia gas-sensing response and repeatability.

**45**

**Figure 11.**

*ZnO Nanorods for Gas Sensors*

**4.2 Flowerlike structures**

temperature.

**4.3 Multishelled hollow spheres**

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

Rather vertically aligned ZnO rods with flowerlike structures synthesized via carbothermal reduction vapor phase transport (CTR-VPT) method exhibited good crystallinity with preferential c-axis orientation and considerable quantity of oxygen vacancy [27]. **Figure 11** shows the ZnO nanorods have diameter in the range of 300–500 nm and length in the range of 7–9.5 μm. In this configuration, a porous network formed by nanorods consists of directional channels for gas diffusion in and out. The interconnected nanorods provide a continuous electrical path for carrier transport between the two gold electrodes. The flowerlike bundle of rods increases the effective surface area and thus enhances gas sensitivity. The H2S sensor with the ZnO nanorods of the flowerlike structure exhibits a high response (e.g., S = 296 at 1 ppm and 581 at 5 ppm) and good selectivity at room temperature and 250°C. However, the response and recovery times decreased with the increasing

ZnO with multishelled hollow spheres of 5 mm in diameter were prepared by a facile solvothermal process in a ternary solvent system. The anisotropic singlecrystalline nanorod building blocks with uniform diameter of about 40 nm are highly directional in the as-synthesized products. The evolution process of hollow ZnO core-shell structures consists of a two-step self-assembly process and symmetric Ostwald ripening starting from the inner core and at the interface of the core and shell. The hollow spheres and core-shell structures could be obtained by simply adjusting the composition of the mixed solvents, as is critical to the packing design [28]. In comparison of the hollow structures and nanorods at all working temperatures, the double-wall hollow structures exhibited the highest sensitivity to formaldehyde gas due to the high donor-related (DL) and the low acceptor-related

*(a and b) Typical top view and (c) cross-sectional SEM images of the vertically aligned ZnO rods with* 

*flowerlike structures. Inset of* **Figure 10(a)** *shows magnified image of one of the flowers.*
