**7. Conclusions and future outlooks**

In this chapter, ZnO nanorods for gas sensors were reviewed. ZnO nanorods with various kinds of morphology were synthesized by different fabrication methods. Their synthesis processes including some reaction formula were introduced, and the sensing properties such as fast response, high responsibility, and good stability were described. Furthermore, the growth of ZnO nanorods can be controlled by tuning the reaction parameters. ZnO nanorods were selectively grown on the seed layer, and their morphology was influenced by the rod growth time. The diameter of ZnO nanorods can be regulated by the solution concentration, the growth time, and the seed density. The ZnO nanorods can form different special structures, and their improvement of the sensing properties was explained from a microscopic point of view. For instance, the flowerlike structure of ZnO nanorods increases the effective surface area and thus enhances gas sensitivity. The sensing performance can be effectively improved by doping with transition metal elements, surface functionalization with noble metals, and decoration with Pd or CaO. Finally, the core-shell nanorods composite based on ZnO, hierarchical nanostructure ZnO/MWCNTs composite, and ZnO nanorods-Gr/M hybrid architectures combine the advantages of both components and improve the performance of the sensors. In the future, novel fabrication methods, further modification, and more composites with different materials are perspectives for the sensors based on ZnO nanorods. The sensing properties such as responsibility and stability need to be improved for the practical application. Moreover, the sensing mechanism of the ZnO nanorods sensors is unclear and necessary to be declared. In summary, great progress have been made on the research of ZnO nanorods for gas sensors; however, improvement area still exists for the further study.
