**2.2 Microemulsion synthesis**

Zinc oxide nanorods were prepared by surfactant-assisted microemulsion method. The microemulsion for the synthesis of ZnO nanorod consists of surfactant such as ethyl benzene acid sodium salt (EBS), dodecyl benzene sulfonic acid sodium salt (DBS), and zinc acetate dihydrate (ZnAc2·2H2O) in xylene. Then the mixture solution of hydrazine monohydrate and ethanol was added drop-wisely to the microemulsion at room temperature by simultaneous agitation. After refluxing the resulting precursor-containing mixture and centrifuging the milky white suspension, the precipitate was rinsed and dried [16].

The aspect ratio of ZnO nanorods was affected by the alkyl chain length of surfactant. ZnO nanorods synthesized by EBS with short alkyl chain length show higher aspect ratio than those by DBS. The response of ZnO nanorods to CO in air was strongly affected by the surface area, defects, and oxygen vacancies. Therefore, ZnO nanorods synthesized by the microemulsion synthesis have large aspect ratio and enhanced gas-sensing properties.

### **2.3 Microwave-assisted hydrolysis preparation**

Highly oriented (002) plane-bounded ZnO nanorods ended with a surface defect hexagonal plane were prepared through microwave-assisted hydrolysis and used as a CO gas detector [17]. In the growth process, growth solution was prepared by dissolving zinc nitratehexahydrate (ZnNO3·6H2O) and hexamethylenetetramine

#### **Figure 2.**

*(A) SEM image of ZnONRs synthesized with hydrothermal method at 85°C within Pt electrodes. (B) Higher magnification of ZnO NRs grown from different islands making NR junctions. (C) Image of a single ZnO NR.* (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 CO gas detection system at room temperature.

### **2.4 Gas-solution-solid method**

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 as follows:

$$
\mathfrak{a} \not\simeq \mathfrak{n} \cup \mathfrak{o}\_z \xrightarrow{\sim} \mathfrak{a} \mathfrak{a} \mathfrak{a} \mathfrak{o} \tag{1}
$$

$$\begin{array}{c} \begin{array}{c} \begin{array}{c} \begin{array}{c} \text{\$ } \end{array} \end{array} \end{array} \begin{array}{c} \begin{array}{c} \text{\$ } \end{array} \star \begin{array}{c} \begin{array}{c} \text{\$ } \end{array} \star \begin{array}{c} \text{\$ } \end{array} \end{array} \begin{array}{c} \begin{array}{c} \text{\$ } \end{array} \star \begin{array}{c} \begin{array}{c} \text{\$ } \end{array} \star \begin{array}{c} \text{\$ } \end{array} \end{array} \end{array} \end{array} \end{array} \tag{2}$$

$$
\pi \colon \pi\_{\pi} \colon \mathsf{A} \times \mathsf{n} \colon \mathsf{n} \to \mathsf{A} \tag{3}
$$

$$\{\mathbf{v}\} \sim \mathbf{v} \times \dots \times \mathbf{v} \times \mathbf{v} + \mathbf{v} \times \mathbf{v} \times \mathbf{v} + \underbrace{\mathbf{v}}\_{\mathbf{v}} \times \mathbf{v} \times \dots \times \mathbf{v} + \mathbf{v} \times \mathbf{v} \times \mathbf{v} \times \mathbf{v} \}$$

$$\text{הייכייל}\left(\text{''},\text{''},\text{''},\text{''},\text{''}\right) \begin{array}{c} \begin{array}{c} \text{''} \ \text{''} \ \text{''} \ \text{''} \ \text{''} \end{array} \right) \tag{5}$$

**39**

**Figure 4.**

*The scheme of the spray pyrolysis setup.*

*ZnO Nanorods for Gas Sensors*

**2.5 Spray pyrolysis**

**2.6 Sonochemical route**

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

intensity reduced to 50% of maximum.

According to the Bravais-Donnay-Harker law, once nucleate, crystal planes with smaller dhkl values grow faster, and the ZnO growth along [0001] is much faster than that along other directions. Ammonia in the solution acts as a transporter of Zn2+ ions (**Figure 3**-3). Finally, ZnO nanorod arrays are formed on Zn substrates (**Figure 3**-4). The sizes of ZnO nanorod arrays could be controlled by tuning the reaction time and the concentration of the ammonia aqueous solution. ZnO nanoarray sensor has both high sensitivity to ammonia and reversibility at room temperature (25°C). And the response could be kept at least 5 days when the current

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.

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].

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

(6)

**Figure 3.** *Schematic diagram showing the ZnO nanorod arrays growth.*

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

According to the Bravais-Donnay-Harker law, once nucleate, crystal planes with smaller dhkl values grow faster, and the ZnO growth along [0001] is much faster than that along other directions. Ammonia in the solution acts as a transporter of Zn2+ ions (**Figure 3**-3). Finally, ZnO nanorod arrays are formed on Zn substrates (**Figure 3**-4). The sizes of ZnO nanorod arrays could be controlled by tuning the reaction time and the concentration of the ammonia aqueous solution. ZnO nanoarray sensor has both high sensitivity to ammonia and reversibility at room temperature (25°C). And the response could be kept at least 5 days when the current intensity reduced to 50% of maximum.
