“…Jiao et al 34 synthesized three different ZnO nanostructures, including dense nanorods, dense nanowires, and sparse nanowires, for gas sensors and found that the sparse nanowires are effective for the detection of NO 2 . Liu et al 24 prepared vertically aligned ZnO nanorods for NO 2 gas sensors, and the response value was reported to be approximately 200 toward 5 ppm NO 2 at 250 C. Zhou et al 35 used ZnO nanoowers for SO 2 sensors, and a response value of 15 was reported for 30 ppm SO 2 at an optimum working temperature of 260 C. The ZnO, and CdO-ZnO nanorices were prepared for formaldehyde gas sensing applications, 36 where the materials have a big size with a length and diameter of about 3 mn and 130 nm, respectively, without the hollow structure, leading to low adsorption sites for gas adsorption. Clearly, the gas sensing properties of ZnO are determined by its (i) crystal size, (ii) porosity, (iii) assembly, and (iv) characteristics.…”