**4.2 Flowerlike structures**

*Nanorods and Nanocomposites*

**Figure 10.**

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

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

**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

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 configu-

when ethanol percentage in solvent increased from 10 to 50%.

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

**4.1 Cross-linked configuration**

very sharp response and recovery time of 20 and 8 s, respectively [25].

rations shows good ammonia gas-sensing response and repeatability.

**44**

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 temperature.
