Selective Oxidizing Gas Sensing and Dominant Sensing Mechanism of n-CaO-Decorated n-ZnO Nanorod Sensors

Abstract

In this work, we investigated the NO and CO sensing properties of n-CaO-decorated n-ZnO nanorods and the dominant sensing mechanism in n-n heterostructured one-dimensional (1D) nanostructured multinetworked chemiresistive gas sensors utilizing the nanorods. The CaO-decorated n-ZnO nanorods showed stronger response to NO than most other ZnO-based nanostructures, including the pristine ZnO nanorods. Many researchers have attributed the enhanced sensing performance of heterostructured sensors to the modulation of the conduction channel width or surface depletion layer width. However, the modulation of the conduction channel width is not the true cause of the enhanced sensing performance of n-n heterostructured 1D gas sensors, because the radial modulation of the conduction channel width is not intensified in these sensors. In this work, we demonstrate that the enhanced performance of the n-CaO-decorated n-ZnO nanorod sensor is mainly due to a combination of the enhanced modulation of the potential barrier height at the n-n heterojunctions, the larger surface-area-to-volume ratio and the increased surface defect density of the decorated ZnO nanorods, not the enhanced modulation of the conduction channel width.

ZnO Nanorods for Gas Sensors

“…The Pd-decorated ZnO nanorods showed a stable performance as well as good stability and repeatability. The response time was approximately 12 min, but the recovery time was much longer, approximately 10 h. Moreover, the CaO-decorated n-ZnO nanorods showed stronger response to NO 2 than the pristine ZnO nanorods [34].…”

ZnO Nanowire Application in Chemoresistive Sensing: A Review

“…While, in biosensors, different analytes can be targeted with a special biomolecule, it is an especially difficult task to differentiate among the same type of gases (oxidizing or reducing), since depletion layer width change and consequently response will be in the same direction. Selective recognition of the gases can be addressed by variations in chemical adsorption and dissociation of the target gases at the NW surface; therefore, NW sensor selectivity can be approached by several methods: NW geometry control [ 36 , 37 ], NW functionalization [ 38 , 39 ], selective contact formation [ 40 ], heterojunction [ 41 ], operating temperature modulation [ 42 ], and sensor array formation [ 43 ].…”

Selective Detection of a Reducing Gas Using WO3‐Decorated ZnO Nanorod‐Based Sensor in the Presence of Oxidizing Gases

“…The judicious choice of two appropriate n‐type materials to form such heterojunctions is important to optimize sensor performance and hence, this aspect has been intensely researched by many groups. Recently, we reported that in the case of a nanosensor with n–n heterostructure, if the workfunction of the decorating material was lower than that of the decorated material, the sensor showed higher response to oxidizing gases as compared to reducing gases; for example, the response of a CaO‐decorated ZnO NR sensor to NO 2 was stronger than its response to CO . We also predicted that conversely, if the workfunction of the decorating material is higher than that of the decorated material, the sensor should show higher response to reducing gases.…”

“…However, despite various advantages such as simple fabrication methods, low cost and high compatibility with other processes, the room temperature performance of semiconducting metal‐oxide (SMO)‐based sensors is currently far from satisfactory . To further improve the performance of such sensors, several techniques including doping with a noble metal or forming heterostructures have been developed. Because of their better response to analyte gases and stability in low oxygen environments, n‐type SMOs are generally much more widely used as base materials for gas sensors as compared to p‐type SMOs .…”

“…In recent years, Schottky type junction gas sensors have attracted particular attention because of their excellent sensing performances. The formation of a Schottky type junction is commonly achieved by decorating n‐type SMO nanorods (NRs) or thin films with another n‐type SMO nanoparticle, by simply mixing the two constituents, or by forming core–shell nanostructures . In this study, the n–n heterojunction was obtained by decorating ZnO NRs with WO 3 nanoparticles.…”