**2. Fabrication methods of ZnO nanorods**

### **2.1 Hydrothermal method**

A two-step low-temperature hydrothermal method was adopted to synthesize ZnO nanorods (NR) selectively grown on-chip as shown in **Figure 1**. The electrodes of Pt on Cr were fabricated by sputter deposition and lift-off technique on a glass substrate (**Figure 1(A)**). On a glass substrate, small zinc islands were sputter deposited as nucleation sites for the ZnO nanorod growth (**Figure 1(B)**). In the next two steps, an equimolar solution of Zn (NO3)2·6H2O and (CH2)6N4 was used. The solution was used for growth nucleation of short ZnO nanorods in the first step (**Figure 1(C)**), whereas it was used for the nanorods elongation in the second step (**Figure 1(D)**). Nanorod junctions were formed by the connection of long porous nanorods from neighboring islands.

#### **Figure 1.**

*Formation of ZnO NRs junctions by two-step hydrothermal growth: (A) deposition of Pt electrodes, (B) deposition of Zn seed islands, (C) growth of dense ZnO nanorods (first step), and (D) growth of long porous ZnO nanorods (second step).*

**37**

**Figure 2.**

*ZnO Nanorods for Gas Sensors*

to NO2 was the highest [15].

**2.2 Microemulsion synthesis**

suspension, the precipitate was rinsed and dried [16].

**2.3 Microwave-assisted hydrolysis preparation**

and enhanced gas-sensing properties.

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

**Figure 2(A)** shows the SEM images of the sensing part of the ZnO NR sensor. As shown in **Figure 2(A)** and **(B)**, the isolated islands were designed as nucleation sites for creating NR junctions within the conducting path from one electrode to the other of the sensor. ZnO NRs in **Figure 2(C)** is a porous material formed from nanocrystals, and its surface is very rough. The measurement of the nanorod gas sensor toward NO2, ethanol, hydrogen, and ammonia indicated that the sensitivity

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

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

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

*(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.*

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

*Nanorods and Nanocomposites*

ethanol 500 ppm at 360°C [12].

**2.1 Hydrothermal method**

nanorods from neighboring islands.

**2. Fabrication methods of ZnO nanorods**

both oxidative and reductive gases [6, 7]. Since ZnO thin film was used to detect gaseous components at about 400°C for the first time in 1966 [8], ZnO has been widely investigated due to its good and stable gas-sensing properties. Various ZnO morphologies provide different sensing performances [9]. For instance, ZnO nanopetals have a sensitivity of 119–20 ppm NO2 gas at room temperature [10]. Monodisperse ZnO hollow six-sided pyramids have a sensitivity of about 15 to dimethylformamide (DMF) and 187 to ethanol [11]. The flowerlike ZnO nanostructures showed the highest response of 144.38 and excellent selectivity to

Gas sensors based on one-dimensional ZnO nanostructures have recently attracted much attention due to its high sensitivity and low power consumption [13, 14]. Especially ZnO nanorods have been widely used for detecting lowconcentration gases due to its range of conductance variability, response toward both oxidative and reductive gases, and highly sensitive and selective properties. In this chapter, the fabrication methods of ZnO nanorods, their controllable growth, their different configurations, their modification for improving sensing

property, and their composites for gas sensors are thoroughly introduced.

*Formation of ZnO NRs junctions by two-step hydrothermal growth: (A) deposition of Pt electrodes, (B) deposition of Zn seed islands, (C) growth of dense ZnO nanorods (first step), and (D) growth of long* 

A two-step low-temperature hydrothermal method was adopted to synthesize ZnO nanorods (NR) selectively grown on-chip as shown in **Figure 1**. The electrodes of Pt on Cr were fabricated by sputter deposition and lift-off technique on a glass substrate (**Figure 1(A)**). On a glass substrate, small zinc islands were sputter deposited as nucleation sites for the ZnO nanorod growth (**Figure 1(B)**). In the next two steps, an equimolar solution of Zn (NO3)2·6H2O and (CH2)6N4 was used. The solution was used for growth nucleation of short ZnO nanorods in the first step (**Figure 1(C)**), whereas it was used for the nanorods elongation in the second step (**Figure 1(D)**). Nanorod junctions were formed by the connection of long porous

**36**

**Figure 1.**

*porous ZnO nanorods (second step).*

**Figure 2(A)** shows the SEM images of the sensing part of the ZnO NR sensor. As shown in **Figure 2(A)** and **(B)**, the isolated islands were designed as nucleation sites for creating NR junctions within the conducting path from one electrode to the other of the sensor. ZnO NRs in **Figure 2(C)** is a porous material formed from nanocrystals, and its surface is very rough. The measurement of the nanorod gas sensor toward NO2, ethanol, hydrogen, and ammonia indicated that the sensitivity to NO2 was the highest [15].
