We have investigated nitrogen monoxide (NO) gas sensing properties utilizing a p-n heterojunction made by the n-type zinc oxide and p-type copper oxide. Our attention has been devoted to the detection of the nitric oxide or nitrogen monoxide (NO) gas among the poisonous gases. We have expected 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. We synthesized the ZnO thin films (ZnO TF) and CuO thin films (CuO TF) and CuO nanoparticles (NPs) by spin coating method on indium tin oxide (ITO) substrates. on the other hand, ZnO nanorods (ZnO NRs) was fabricated by a hydrothermal method.
The crystallinity and microstructure of 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 TF/CuO TF p-n heterojunction gas sensor exhibited a maximum sensitivity to NO gas in dry air at an operating temperature of 100 ℃ and increase linearly with the increase in the NO gas concentration from 10 to 30 ppm. The current-voltage (I-V) characteristics of the CuO NPs/ZnO NRs p-n heterojunction and its temperature dependence ranging from room temperature (RT) to 240 ℃ 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 3 to 10 ppm in dry air. The NO sensitivity value of the p-n heterojunction has been observed to be as high as 265 % at RT 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 as a promising candidate for NO gas detection.