We have investigated nitrogen monoxide (NO) gas sensing properties of n-type zinc oxide nanorods (ZnO NRs) and copper oxide thin film (CuO TF)/ZnO NRs p-n heterojunction structure with n-type zinc oxide nanorods (ZnO NRs)
embedded in p-type copper oxide thin film (CuO TF), expecting that the formation of electron depletion layers in the p-CuO and n-ZnO could lead to an enhancement of the adsorption of a target gas. Our attention has been devoted to the detection of the nitric oxide or nitrogen monoxide (NO) gas among the poisonous gases. We synthesized the ZnO NRs by a hydrothermal method using a ZnO nanoparticle seed layer deposited on Si/SiO2 and indium tin oxide (ITO) substrates by sputtering method. The p-CuO TF/n-ZnO NRs heterojunction has been fabricated by employing ZnO NRs synthesized on ITO and CuO TF deposited onto the ZnO nanorod array by sputtering method. The crystallinity and microstructure of the ZnO NRs and heterojunction materials have been examined by an X-ray diffractometer (XRD) and scanning electron microscope (SEM). It was found from the gas sensing measurements that the ZnO NRs gas sensor exhibited a maximum sensitivity to NO gas in dry air at an operating temperature of 230 ℃ and increase linearly with the increase in the NO gas concentration from 2 to 18 ppm. The current-voltage (I-V) characteristics of the CuO TF/ZnO NRs p-n heterojunction and its temperature dependence ranging from room temperature (RT) to 250 ℃ have been examined in dry air and NO ambient, revealing a nonlinear rectifying diode behavior. When the oxide p-n junction was exposed to an oxidizing or electron acceptor gas NO, we have observed a significant reduction in the forward current of the heterojunction, which can be explained by the generation of holes inside the p-type CuO layer. The NO gas response of the oxide heterojunction has been found to exhibit a maximum at an operating temperature of 150 ℃ and increase linearly with increasing NO gas concentration from 2 to 14 ppm in dry air. The NO sensitivity value of the p-n heterojunction has been observed to be as high as 1000 % at 150 ℃ when biased at 2 V in the presence of 10 ppm NO. Furthermore, the oxide heterostructure gas sensor has shown a stable and repeatable response to NO gas. These experimental results illustrates the fabrication of the p-n oxide heterojunction structure with n-type ZnO NRs embedded in p-type CuO TF as a promising candidate for NO gas detection.