Fabrication And Gas Sensing Properties Of CuO-ZnO Hetero-structured Nanomaterials

Gas sensing has important applications in the fields of industrial production,environmental monitoring & protection,aerospace technology,smart home system,medical treatment and so on.In particular,metal oxide semiconductor nanomaterials,such as ZnO,CuO and SnO₂ nanomaterials,have become considerably attractive for gas sensing due to the large surface area /volume ratio,low cost,various preparation methods,as well as high sensitivity and stability of gas sensing properties.The effects of types,morphologies,crystal structures,preparation methods,post-treatments,doping,and surface modification of metal oxide nanomaterials on their gas sensing properties and mechanisms have been extensively studied.However,to further enhance the gas sensing properties and to meet the growing market demand have stimulated considerable interest in exploration of novel or optimized gas-sensing oxide nanomaterials and improvement in understanding of their gas sensing mechanisms.In this present work,fabrication and gas sensing properties of several CuO-ZnO hetero-structured nanomaterials have been studied extensively.Pulsed laser deposition?PLD?and solvothermal method have been employed for sample preparation.Influence of the growth parameters on properties and growth mechanisms of the samples has been revealed.Importantly,electrical or optical gas sensing properties of ZnO nanorods,two-dimensional ultra-thin ZnO nanosheets,CuO-ZnO hetero-structured nanorods and CuO-ZnO hetero-structured nanosheets have been investigated systematically by using oxygen,hydrogen sulfide?H2S?,ethanol as the target gases.The underlying sensing mechanisms have been discussed.The thesis includes three sections:Firstly,effects of incident laser fluence on morphologies,crystal defects and photoluminescence?PL?properties of the ZnO nanorods produced by PLD method have been studied extensively.Influence of environmental temperature and gases on PL properties of the products has been investigated as well.Furthermore,optical oxygen sensing properties of the ZnO nanorods have been explored by monitoring intensity of the near-band gap emission at ³80 nm.The effects of morphologies and crystal defects of the ZnO nanorods on the PL-based oxygen sensing properties have been studied and the underlying mechanisms have been discussed accordingly.High oxygen-sensing response,recoverability and reproducibility as well as fast response and recovery times were achieved at a working temperature of 150 °C,attributed to both the high S/V ratio of the PLD nanorods and the progressive desorption of competing pre-adsorbed specieswith increasing working temperature.Secondly,novel CuO nanoparticle-capped ZnO nanorods have been produced using a co-deposition PLD method.These nanorods are shown to grow by a CuO-nanoparticle-assisted vapor-solid-solid mechanism.The near-band edge emission intensity of these CuO-ZnO hetero-structured nanorodsshows large variations upon exposure to trace quantities of H2S gas.A high response of 78% for 15 ppm H2S has been achieved together with good recoverability and reproducibility.This study represents the first demonstration of PL-based H2S gassensing,at room temperature,with sub-ppm sensitivity.The related sensing mechanisms have been revealed that both the Cu-doped ZnO stem and the CuO capping nanoparticle contribute to optical H2S sensing with these CuO-ZnO nanorods.Finally,two-dimensional?2D?ZnO nanosheets produced by a solvothermal method were demonstrated to present high sensing performance to ethanol.After decoration by pulsed laser ablation of CuO target,the sensing response of the resulted CuO-ZnO nanosheets to ethanol in the concentration of 1-2000 ppm was revealed to be further enhanced,e.g.CuO decoration resulted in response enhancement up to 70.5% for 100 ppm ethanol.Moreover,shortened recovery time as wellas the high ethanol-sensing recoverability and reproducibility have been also demonstrated.Both the 3D framework of the 2D ZnO nanosheets and the formation of CuO-ZnO p-n junctions have been suggested to contribute to the high sensing performance of the CuO-ZnO hetero-structured nanosheets.Novel CuO-ZnO hetero-structured nanomaterials have been successfully fabricated and their excellent gas sensing properties as well as the underlying gas-sensing mechanisms have been revealed.This work may benefit practical gas sensor application of CuO-ZnO hetero-structured nanomaterials.