**4.4 Hollow microhemispheres**

Self-assembling ZnO nanorods into 3D ZnO hollow microhemispheres is simply done by solvothermal reaction of zinc and alkali source in the presence of ethylene glycol. The 3D ZnO hollow microhemispheres were synthesized by the primary formation of glycolate precursors and subsequent transformation into ZnO. Glycol such as EG and the solvothermal process play the critical role in the synthesis. Numerous orderly and radical nanorods in the 3D ZnO hollow microhemispheres have the diameter of about 50 nm and length of several hundred nanometers. Furthermore, high sensitivity for ethanol and ammonia as well as quick response and recovery time at room temperature were obtained for the 3D ZnO hollow microhemisphere-based gas sensor due to the high surface-to-volume ratio [29].
