Tailored nanostructures for CO2 gas sensing applications
Creators: name: Joshi, Shravanti
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 401807
subject: Nanomaterials
classificationCode: 401810
subject: Nanoscale characterisation
classificationCode: 410306
subject: Environmental nanotechnology and nanometrology

description: Carbon dioxide (CO2) is a thermodynamically stable gas owing to its inherent colourless, odourless and inflammable nature. At low concentrations (~1000 ppm) it is considered harmless, however at higher concentrations (&gt;20000 ppm), it significantly affects the respiratory system. In recent years, the health hazards of CO2 have forced government and environmental safety bodies around the world to introduce stern rules to curb CO2 emission from major anthropogenic sources. In this context, efficient and selective detection of CO2 gas has gained considerable interest as it is the first step to emission reduction of this greenhouse pollutant. To date, metal oxide based sensors have shown promising potential in gas sensing applications as they can be easily miniaturized, has defect tuned facile synthesis and high structural stability necessary in harsh testing conditions. However, the major issue of using them for CO2 sensing applications is their lack of sensitivity and selectivity towards the gas at the desired operating temperatures. This doctoral project centres around the development of metal oxides based CO2 gas sensors that can detect low concentrations (&lt;1000 ppm) in the presence of oxidising co-interfering gases such as NO2, SO2, CO and NO at various operating temperatures.<br><br>The initial stage of this doctoral work deals with comprehensive literature review that revealed challenging inadequacies in the existing sensor materials synthesis and fabrication techniques. Furthermore, important semiconductor materials that have not yet been explored towards CO2 but have shown sensing capabilities towards similar oxidising gases were studied for the first time. In an attempt to address the research questions developed during the course of study, potentiometric and chemo-resistive micro-sensors were fabricated as a case study to successfully develop a set of sensitive and selective CO2 gas sensors operating from 100 to 500oC for range of potential applications. Among the CO2 sensors, potentiometric sensors are extensively tried in sectors requiring sensors operating efficiently at temperature above 400oC. Unlike the conventional potentiometric sensors using commercially procured sensitive materials, the sensor developed in this work was fabricated out of nanomaterials in a bid to improve its electrochemical stability. In case of chemo-resistive gas sensors, various hierarchical n-type metal oxide semiconductors (ZnO, SnO2 and BaTiO3) were synthesized and decorated with Ag@CuO to target CO2 application, where an operating temperature less than 300oC is required. The chosen n-type metal oxides were selected based on their band-gap, work function, and charge carrier concentration. Carbonising ability of CuO in the presence of CO2 and catalytic role of Ag in expediting the CO2 sensing phenomenon were the sole reasons for the decoration of n-type MOS with Ag@CuO. Plausible formation mechanism in each case was proposed based on the nucleation and crystal growth supported by physicochemical characterizations techniques. Both types of the developed sensors were tested towards CO2 gas concentrations (100-10000 ppm) as a function of operating temperature (40-700°C) and their influence on sensor performance (i.e. sensitivity, selectivity, long-term stability, response/recovery times, dynamic range, repeatability and reusability) were thoroughly investigated. The probable CO2 sensing mechanism has also been attempted by corroborating sensing performance with in-situ spectroscopic techniques.<br><br>The results demonstrated in this work were found to be encouraging and is anticipated to pave way for intensified research efforts in CO2 sensor development for real industrial applications. For example, the nanostructured potentiometric CO2 composing of Li4Ti5O12 octahedra showed enhanced response compared to commercial sensor, thus standing strong chance as real-time sensor. Among the n-type MOS tested, Ag@CuO/BaTiO3 showed exceptional sensing performance and the sensing mechanism were determined by employing in-situ DRIFTS technique. The sensors developed in this doctoral work are anticipated to be suitable for industrial applications requiring low operating temperatures and detection limits. The methods, materials and devices presented in this thesis have potential to be explored further, not only for CO2 based sensing but also many other applications such as visible light driven dye degradation, supercapacitors and so on.