**5.1 Doping**

The ZnO-sensing performance can be effectively improved by doping with transition metal elements. Vertically aligned yttrium-doped ZnO nanorod (YZO NR) arrays were synthesized by a one-pot hydrothermal method [30]. The Y doping concentration strongly influences the surface morphology of the NRs. With the dopant concentration increasing, longer and sharper NRs with high aspect ratio formed, and the aspect ratio of the YZO NRs was increased from 11 to 25. The Y doping reduced the ZnO NR resistivity to its minimum by a factor of 115 for the Y0.10ZnO. And the corresponding sensor gave the highest sensitivity toward all the tested gas species with the lowest breakdown voltage. The sensitivity of the sensor based on Y-doped ZnO nanorod arrays was enhanced up to sixfold, and the breakdown voltage from the highly conductive YZO NRs significantly reduced. The Cr-doped ZnO sensor presents the response of 104 to 100 ppm acetone at 300°C, as is four times higher than that of ZnO sensor. The increase of electron concentration optimized by Cr doping improved the sensing response and selectivity [31].
