Super rapid response of humidity sensor based on MOCVD grown ZnO nanotips array

This work deals with the fabrication of a-plane ZnO nanotip based humidity sensor on r-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD). The humidity sensing property has been studied for this sensor by impedance measurements. The impedance of the humidity sensor decreases by about six orders of magnitude (from 2.8 × 108 Ω to 4.3 × 102 Ω at 1 V, 100 Hz) with increase in relative humidity (RH) (from 19% to 95%). The response and recovery times were found to be 3 and 12 s, respectively. It shows super fast response and recovery time with good selectivity to humidity. Considering different sensor parameters investigated in the present study, it is observed that MOCVD grown ZnO nanotips respond promptly to the changes in RH due to quantum size effect as is evident from the blue shift of its band gap; and high atomic density on the a-plane of ZnO. The response has been explained on the basis of the adsorption of water molecules on a-plane ZnO and proton transport mechanism.