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Title: Development of nanomaterial-based gas microsensors for environmental application
Author: Liu, Lin
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2019
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Air pollution is a serious environmental problem that jeopardizes human' health. Inhalation of gas pollutants can cause many health problems such as respiratory disease, lung cancer, leukemia, or even death. Thus, development of new technologies and novel portable miniaturized sensors to detect trace gas pollutants has received significant attention. This study aims to develop high performance gas sensors by designing hierarchical nanostructured materials and new device structures. In Chapter 2, novel brush-like (B-) ZnO@SnO2 n-n hierarchical nanostructures (HNSs) were successfully synthesized by using a simple two-step hydrothermal method. The growth mechanism of the B-ZnO@SnO2 HNSs was studied. The B-ZnO@SnO2 HNS-based NO2 sensor showed high sensing performance including high sensitivity (response value: 25.5 to1 ppm NO2), fast response (< 60 s), and low detection limit (5 ppb). The enhanced sensing mechanism was attributing to the unique structure of B- ZnO@SnO2 HNSs, which can provide large specific surface area and can induce synergism effect due to the formed multi-junctions. In Chapter 3, NiO nanowalls decorated by SnO2 nanoneedles (NiO@SnO2) were directly grown on ceramic chips via a chemical bath deposition method to obtain uniform sensing materials without aggregation instead of using slurry-coating method, which is used in the Chapter 2. The morphologies and compositions of the NiO@SnO2 HNSs were well tuned by varying the growth time to optimize the sensing performance. The response of the NiO@SnO2 HNSs (2 h) to 1 ppm H2S was over 23 times higher than that of the pure NiO nanowalls and 17 times higher than that of the pure SnO2 nanosheets. This dramatic enhancement is mainly due to the large surface area of the NiO@SnO2 HNSs and the p-n heterojunction at the heterointerface of SnO2 and NiO. The variation in the depletion layers (W_SnO₂ and W_NiO) at the interface of SnO2 and NiO greatly depends on the properties of the target gases (e.g., electronwithdrawing property (NO2) or electron-donating property (H2S)). In Chapter 4, based on the in situ growth method ("bottom-up") which was reported in Chapter 3, a "top-down" and "bottom-up" combined strategy was proposed to manufacture wafer-scale miniaturized gas sensors with high-throughput by growth of patterned Ni(OH)2 nanowalls at specific locations. First, micro-hotplates were fabricated on a two-inch (2") silicon wafer by micro-electro-mechanical-system (MEMS) techniques ("top-down" strategy). Then a template-guided controllable de-wetting method was used to assemble a thermoplastic elastomer (TPE) thin film with uniform micro-sized holes (relative standard deviation (RSD) of the size of micro-holes < 3.5 %, n >300), which serves as the mask for growing Ni(OH)2 nanowalls ("bottom up" strategy). The obtained sensors based on these strategies showed great reproducibility of electric properties (RSD < 0.8%, n=8) and sensing performance toward H2S (RSD <3.5%, n=8). Different from the chemiresistive gas sensors reported in this thesis, which are driven by external power source, a novel device structure was proposed to construct photovoltaic selfpowered H2S sensor based on p-type single-walled carbon nanotubes (SWNTs) and n-type silicon (n-Si) heterojunction. The energy from visible light suffices to drive the device owing to a built-in electric field (BEF) induced by the differences between the Fermi energy levels of SWNTs and n-Si. Under 600 nm illumination (1.8 mW/cm2 ), linearly self-powered detecting H2S in the range of 100 ppb to 800 ppb was implemented, with fast response time (57 s) and recovery time (110 s) at room temperature. Compared with conventional chemiresistive sensor based on SWNTs, the response time and response of the photovoltaic self-powered device were significantly enhanced. When exposing to 400 ppb H2S, the sensing response increased more than 4 times attributed to the BEF. Overall, three different heterostructure-based gas sensors were studied to enhance their performance and the sensing mechanisms of the sensors were also investigated. Moreover, a novel "top-down" & "bottom-up" strategy was proposed for wafer-scale fabrication of miniaturized gas sensor.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral