Geometric Effect of ZnO Nanorods on Ethanol Sensing Properties: The Relative Role of a Seed Layer

Abstract

ZnO nanorods were grown on ZnO seed layers, i.e. thin films deposited on flexible polyimide substrates. The ZnO seed layers were prepared by rf sputter deposition and the ZnO nanorods were prepared by a thermolysis assisted aqueous solution method. Structural characterizations show that the seed layer thickness, less than 200 nm, does not affect the growth behavior or the crystalline phase of ZnO nanorods. The sensitivity of both ZnO thin film and nanorods sensors to ethanol were compared as a function of the thin film (seed layer) thickness. The highest sensitivity for the ZnO thin film sensors was observed at a film thickness of 40 nm but no effect was seen from the film (seed) thickness on the ZnO nanorod sensor sensitivity. The nanorod dimensions obviously played an important role in determining the sensitivities. These results indicate that the charge carrier density within the ZnO nanorods and transport through the nanorods are primarily responsible for the gas sensing properties. A quantitative equation was derived to explain why control of the ZnO nanorod dimensions is more effective than control of the seed layer's thickness regarding better gas sensitivity.