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UBC Theses and Dissertations

Design, fabrication and evaluation of photo-activated gas sensors based on semiconducting nanostructures Espid, Ehsan


In this research, UV-LED-activated metal-oxide gas sensors were studied, where the electrochemical properties of the sensing material were enhanced by manipulation of sensor operating parameters as well as material structure and composition. In the first phase, the sensing characteristics of several favorable metal oxide semiconductors were investigated under different wavelengths, irradiances, and pulsation frequencies. The results showed that UVA-LEDs (i.e. 365 nm) could result in better responses due to resonant absorption. At a constant photon energy (i.e. 3.4 eV), increasing irradiance enhanced the photo-desorption of adsorbed components and reduced the response. Pulsed UV irradiation could significantly enhance the response as a result of increased residence time of adsorbates. Also, a set of strategies were applied to investigate and modify the sensing layer structure and composition. Firstly, Ag was incorporated into ZnO nanoparticle structure which resulted in an increase in sensor response toward 5 ppm NO₂ (ΔR/R = 0.98), compared to pristine ZnO (0.5) potentially due to layer charge carrier enhancement. Secondly, a sensing material structure was designed based on ZnO nanowires decorated with Pt nanoparticles, as metallic co-catalytic sites. The ZnO nanowires showed an increased response (1.6) compared to nanoparticles due to their higher surface area. Furthermore, decorating the surface of ZnO nanowires with Pt nanoparticles remarkably enhanced the sensing performance, whereas 0.1 wt% Pt decorated ZnO nanowires sensor exhibited nearly 4-times higher and 50s faster response compared to ZnO nanoparticles, in identical photo-activation settings. Thirdly, ZnO nanowires were used as a core for a thin layer of a secondary semiconductor to develop ZnO-In₂O₃ and ZnO-SnO₂ core-shell gas sensors. The relative responses of the ZnO-In₂O₃ (2.49) and ZnO-SnO₂ (2.21) sensors were higher than that of pristine ZnO NWs (1.6), possibly due to the improved photon absorption, and increased active sites on the surface of the nanowires. Lastly, carbon mesoporous materials (CMMs) with various ZnO loading concentrations were tested against NO₂ and NH₃, where the sensors responded 1.91 (NO₂) and 1.35 (NH₃) in optimal loading concentrations. This improvement in response could be attributed to the high surface area, extended separation, and more oxygen vacancies (OVac) induced by C-dopant in ZnOx/CMM sensors.

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