Direct Growth Of ZnO/SnO₂ Nanosheets/Nanowires On Sensor Substrates And Their Sensing Properties

Metal oxide semiconductors like ZnO and SnO2 have been investigated for several decades and widely adopted to fabricate gas sensors, which exhibit good sensing properties to target gases..However, these polycrystalline film sensors cannot be used for wide practical applications because of their low sensitivity and high working temperature. Recently, many studies show that 1D or 2D ZnO and SnO2 nanostructures, such as nanowires or nanosheets exhibit good gas sensing performances due to their large surface-to-volume ratio, and extensive studies have also been done in improving the performances by adding catalysts or forming semiconductor heterojunction.This thesis mainly focuses on the in-situ direct growth of ZnO and SnO2 nanomaterials, such as nanosheets and nanowires on Al2O3 tube or flat substrate. Many efforts have also been done to enhance their sensing performances by introducing the depletion layer of PN heterojunction or metal/semiconductor Schottky contact. The main results are as follows:1. ZnO and SnO2 nanosheets or ZnO nanowires have been grown directly on the Al2O3 tube or flat substrate by introducing a seed layer, which provides a material platform for gas sensors. It can be realised through a facile one-step hydrothermal method which simplifies the gas sensor fabrication process. We got the better samples with larger surface area and density by optimizing the growth conditions.2. In order to improve the sensing properties, we designed NiO/ZnO and NiO/SnO2 PN heterojunction gas sensors and then verified the enhanced sensing mechanism with PN depletion layer. By employing PLD method, the construction of NiO/ZnO or NiO/SnO2 PN heterojunction is highly controllable and reproducible. In comparison with ZnO nanosheet, the NiO/ZnO nanoparticle/nanosheet heterojunction exhibit higher sensitivity and selectivity to TEA gas. The as-prepared sensor response could get up to 185 to 100 ppm TEA, which is 2.4 times higher than that of ZnO nanosheets. The response of NiO/SnO2 gas sensor is also much higher than that of pristine SnO2 hollow spheres.3. The sensing property was also enhanced by loading noble metal nanoparticles or forming Schottky contact on the surface of SnO2 nanosheets. Gas-sensing characterization demonstrated enhanced TEA sensing performance of SnO2 nanosheets after decoration with Au nanoparticles. The as-prepared Au/SnO2 sensor exhibited faster response-recovery rate and super high response. The response could get up to 400 to 5 ppm TEA with the working temperature of 180℃, which is about 10 times higher than that of SnO2 nanosheets at 320℃.4. Based on the synergistic working principle of NN heterojunction and Schottky contact, we designed a low power consumption and fast response TEA gas sensor by constructing Au-loaded SnO2/ZnO core-shell nanowires and investigated their sensing properties. When Au nanoparticles are loaded onto SnO2/ZnO nanorods, the gas sensor exhibits low working temperature (40℃) and fast response (1.2 s) to TEA gas. The as-prepared sensor response can get up to 12.4 when exposed to 50 ppm TEA with the working temperature of 40℃, which is about 5 times higher than that of ZnO nanowires sensor.