**3.2 Selective growth**

Selective growth of ZnO nanorod arrays with well-defined areas was developed to fabricate the NO2 gas sensor. The seed layer was created by ink-jetting the seed solution on the interdigitated electrodes. Then, vertically aligned ZnO nanorods were grown by the hydrothermal approach on the patterned seed layer. The effects of the seed solution properties and the ink-jet printing parameters on the printing performance and the morphology of the nanorods were investigated [22].

FESEM images in **Figure 7** show the morphology of patterned ZnO nanorod films. ZnO nanorods are selectively grown on a round grown area with a diameter of 650 μm as shown in **Figure 7(a)**. **Figure 7(b)** and **(c)** present the enlarged edge images of the grown area. In **Figure 7(c)** the ZnO nanorod were selectively grown in a direction perpendicular to the substrate to produce vertically aligned arrays

#### **Figure 6.**

*FESEM images of ZnO thin-film sample ZnO:6 to ZnO:12 (a1, a2, and a3) show cross-sectional view of ZnO:6, ZnO:9, and ZnO:12, respectively. a2, a3, b2, b3, c2, and c3 reveal low- and high-magnification FESEM images of ZnO thin-film sample ZnO:6 to ZnO:12.*

**Figure 7.**

*FESEM images of the selective grown ZnO nanorods micropattern: (a) entire pattern, (b) edges of the grown area, and (c) enlarged images of the nanorods.*

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 sensor.

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 than that with shorter growth time.

#### **Figure 8.**

*FESEM images of the cross section of ZnO nanorods prepared with different rod growth time (a) 2 h and (b) 4 h.*

**43**

**Figure 9.**

*10, 20, 30, 40, and 50 vol% ethanol solvent.*

*ZnO Nanorods for Gas Sensors*

**3.3 Diameter regulation**

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

with NaOH concentration increasing [24].

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

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

*SEM images of ZnO nanorod samples (a) a0, (b) a10, (c) a20, (d) a30, (e) a40, and (f) a50 prepared with 0,* 
