**1. Introduction**

With the development of economy and society, the increasingly serious environmental problems have become the world topics of concern to people. Gas sensors have been widely used in medicine industry and ecology to protect people from harmful gases [1]. Generally, gas-sensing mechanism is founded on the electron transport between adsorbed oxygen species and test gas molecules [2]. The response of sensor is not only determined by the amount of adsorbed oxygen species but also by the specific surface area, structure, active sites, and electron properties of sensing materials.

Metal oxide semiconductor sensors have attracted great attention for a long time because they have advantages such as low cost, online operation, low power consumption, high sensitivity under ambient conditions, and high compatibility with microelectronic processing [3]. Metal oxide gas sensors function through controlling the electrical conductivity upon exposure to gases. A number of metal oxides, such as TiO2, ZnO, SnO2, Fe2O3, WO3, etc., have been employed for gas sensors [4].

Among them, ZnO has been widely used to detect low-concentration gases, such as ethanol, benzene, nitrogen oxide, liquid petroleum gas, and other species due to a wide band gap, low-cost environmentally benign components, nontoxicity [5], good thermal and chemical stability, high-electron mobility, large exciton binding energy, and its range of conductance variability and response toward

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 ethanol 500 ppm at 360°C [12].

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.
