**2.5 Spray pyrolysis**

*Nanorods and Nanocomposites*

CO gas detection system at room temperature.

**2.4 Gas-solution-solid method**

as follows:

(HMT) in deionized water. Subsequently, a seeded-FTO substrate immersed in the growth solution was processed in the microwave oven. The sensor with these ZnO nanorods presented a remarkable response at 81.1% toward 100 ppm CO gas exposure and recovery time of approximately 2.5 min. The microwave-assisted hydrolysis is an excellent approach to fabricate ZnO nanorods used for low-concentration

ZnO nanorod arrays on Zn substrate were prepared by the so-called gas-solution-solid method [18]. The aligned ZnO nanorods on substrates were obtained by putting Zn foils above an ammonia solution. The growth mechanism is studied and proposed as shown in **Figure 3**. The Zn foil is first put above the ammonia solution (**Figure 3**-1). The evaporation and condensation of ammonia solution formed a thin layer on the surface of Zn foil (**Figure 3**-2). At the beginning of ZnO nanorod growth, Zn on the surface of substrate is oxidized by O2 to produce ZnO, as then reacts with NH4·OH to form Zn(OH)2 in the thin layer of ammonium solution. The putative reactions relevant to the synthesis of the aligned upright ZnO nanorods are

(1)

(2)

(3)

(4)

(5)

**38**

**Figure 3.**

*Schematic diagram showing the ZnO nanorod arrays growth.*

ZnO nanorods with different sizes of hexagonal pillar shape have been successfully synthesized by spray pyrolysis technique (SPT). Zinc acetate solution was obtained by dissolving zinc acetate dihydrate in the mixture of methanol and double distilled water. During spray pyrolysis process, the precursor solution droplets close to the preheated substrates thermally decomposed and formed the highly adherent zinc oxide film. During the pyrolytic process, the following reaction takes place.

$$\omega\_{\omega} \circ \omega\_{\omega}, \omega\_{\omega} \circ \omega\_{\omega}, \omega\_{\omega} \circ \omega\_{\omega} \circ \omega\_{\omega} \tag{6}$$

Highly uniform crystalline films were obtained upon the post deposition annealing at 500°C for 1 h in air. The spray pyrolysis setup used is schematically illustrated in **Figure 4**. The thin films comprise well-shaped hexagonal ZnO nanorods with a diameter of 90–120 nm and length of up to 200 nm. The gas-sensing properties of these films toward gases such as ethanol, CO2, NH3, CO, and H2S exposure have been investigated at operating temperature from 30 (room temperature) to 450°C. The ZnO nanorods thin films showed much better sensitivity and stability to H2S gas (100 ppm) at 50°C than the conventional ZnO materials without nanostructures [19].

### **2.6 Sonochemical route**

A sonochemical route provides an effective way to grow vertically aligned ZnO nanorod arrays on a Pt-electrode patterned alumina substrate under ambient conditions [20]. **Figure 5(a)** shows the sensor substrate with the interdigitated

**Figure 4.** *The scheme of the spray pyrolysis setup.*

#### **Figure 5.**

*(a) Photograph of sensor substrate including interdigitated comblike Pt electrodes and a resistive heater. (b) Zn thin-film sputtered sensor substrate. (c) A schematic illustration for the sonochemical growth of vertically aligned ZnO nanorod arrays on a sensor substrate.*

comblike Pt electrodes on the front side and a resistive heater on the back. Upon the deposition of Zn thin film (40 nm) on the interdigitated Pt electrodes using RF sputtering technique as shown in **Figure 5(b)**, the sensor substrate was immersed in a mixed aqueous solution of Zn(NO3)2·6H2O and (CH2)6N4. Ultrasonic waves at an intensity of 39.5 W/cm<sup>2</sup> were introduced in the solution for 1 h. **Figure 5(c)** shows the scheme of the sonochemical growth of vertically aligned ZnO nanorod arrays on the substrate. The ZnO nanorods have the average diameter of 50 nm and length of 500 nm. The gas sensor based on sonochemically grown ZnO nanorod has high sensitivity to NO2 gas with a very low detection limit of 10 ppb at 250°C and short response and recovery time.

#### **2.7 Simple solution route**

Dodecyl benzene sulfonic acid sodium salt (DBS) was used as a modifying agent in a simple solution route to fabricate well-crystallized ZnO nanorods [21]. Zinc acetate dihydrate [Zn(AC)2·2H2O] and DBS with a ratio of 1:8.5 were dissolved in a mixed solvent of ethylene glycol and xylene. Then a hydrazine monohydrate ethanol solution was drop-wisely introduced into the solution. After the reaction completed, the mixture was subsequently heated to boiling point (140°C) and refluxed. The resulting products were cooled down naturally, washed, and finally dried in the vacuum at 70°C. The ZnO nanorods sensors are highly sensitive and selective to TEA at low concentration of 0.001–1000 ppm among the gases of toluene, ethanol, benzene, and acetone. The prepared ZnO sensors to TEA exhibit high selectivity and superior sensitivity with the response of 6–0.001 ppm TEA at 150°C.

#### **3. Controllable fabrication of ZnO nanorods**

#### **3.1 Growth control**

The growth characteristics of the ZnO nanorod arrays (ZNAs) deposited using a wet chemical route were affected by several parameters, such as zinc seed layer morphology, zinc ion concentration, solution pH, deposition time, and growth temperature [7]. The surface and the cross-sectional FESEM images of the ZnO

**41**

**Figure 6.**

*of ZnO thin-film sample ZnO:6 to ZnO:12.*

*ZnO Nanorods for Gas Sensors*

100 ppm NO2 at 175°C.

**3.2 Selective growth**

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

changes as the growth time is prolonged.

nanorod films prepared with different growth times were shown in **Figure 6**. The length/diameter aspect ratio of the ZNAs also increased as the reaction time is extended [**Figure 6(a)** and **(c)**]. With the prolonged deposition time, upright and wider nanorods can be produced, but the nanorods easily merge with each other as they grow longer. The alignment of the nanorods in **Figure 6(a3)**, **(b3)**, and **(c3)**

The length and inter-rod space have important influence on the gas-sensing performance of the devices. The ZnO:6 nanorods sample has small length and quite large spacing between them. However, both the length and the inter-rod spacing of the ZnO:9 nanorods samples are adequate and beneficial for the sensing performance. Nevertheless, for the ZnO:12 sample, the interspaces between nanorods are smallest due to the overlap between the nanorods. The gas sensors with ZnO:9 nanorod samples exhibits a high sensitivity of 3100% toward

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

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

*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* 

performance and the morphology of the nanorods were investigated [22].

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

*Nanorods and Nanocomposites*

intensity of 39.5 W/cm<sup>2</sup>

*aligned ZnO nanorod arrays on a sensor substrate.*

**Figure 5.**

response and recovery time.

**2.7 Simple solution route**

**3.1 Growth control**

comblike Pt electrodes on the front side and a resistive heater on the back. Upon the deposition of Zn thin film (40 nm) on the interdigitated Pt electrodes using RF sputtering technique as shown in **Figure 5(b)**, the sensor substrate was immersed in a mixed aqueous solution of Zn(NO3)2·6H2O and (CH2)6N4. Ultrasonic waves at an

*(a) Photograph of sensor substrate including interdigitated comblike Pt electrodes and a resistive heater. (b) Zn thin-film sputtered sensor substrate. (c) A schematic illustration for the sonochemical growth of vertically* 

the scheme of the sonochemical growth of vertically aligned ZnO nanorod arrays on the substrate. The ZnO nanorods have the average diameter of 50 nm and length of 500 nm. The gas sensor based on sonochemically grown ZnO nanorod has high sensitivity to NO2 gas with a very low detection limit of 10 ppb at 250°C and short

Dodecyl benzene sulfonic acid sodium salt (DBS) was used as a modifying agent in a simple solution route to fabricate well-crystallized ZnO nanorods [21]. Zinc acetate dihydrate [Zn(AC)2·2H2O] and DBS with a ratio of 1:8.5 were dissolved in a mixed solvent of ethylene glycol and xylene. Then a hydrazine monohydrate ethanol solution was drop-wisely introduced into the solution. After the reaction completed, the mixture was subsequently heated to boiling point (140°C) and refluxed. The resulting products were cooled down naturally, washed, and finally dried in the vacuum at 70°C. The ZnO nanorods sensors are highly sensitive and selective to TEA at low concentration of 0.001–1000 ppm among the gases of toluene, ethanol, benzene, and acetone. The prepared ZnO sensors to TEA exhibit high selectivity

The growth characteristics of the ZnO nanorod arrays (ZNAs) deposited using a wet chemical route were affected by several parameters, such as zinc seed layer morphology, zinc ion concentration, solution pH, deposition time, and growth temperature [7]. The surface and the cross-sectional FESEM images of the ZnO

and superior sensitivity with the response of 6–0.001 ppm TEA at 150°C.

**3. Controllable fabrication of ZnO nanorods**

were introduced in the solution for 1 h. **Figure 5(c)** shows

**40**

nanorod films prepared with different growth times were shown in **Figure 6**. The length/diameter aspect ratio of the ZNAs also increased as the reaction time is extended [**Figure 6(a)** and **(c)**]. With the prolonged deposition time, upright and wider nanorods can be produced, but the nanorods easily merge with each other as they grow longer. The alignment of the nanorods in **Figure 6(a3)**, **(b3)**, and **(c3)** changes as the growth time is prolonged.

The length and inter-rod space have important influence on the gas-sensing performance of the devices. The ZnO:6 nanorods sample has small length and quite large spacing between them. However, both the length and the inter-rod spacing of the ZnO:9 nanorods samples are adequate and beneficial for the sensing performance. Nevertheless, for the ZnO:12 sample, the interspaces between nanorods are smallest due to the overlap between the nanorods. The gas sensors with ZnO:9 nanorod samples exhibits a high sensitivity of 3100% toward 100 ppm NO2 at 175°C.
