Investigation of zinc and titanium oxide-based nanoarrays for light assisted acetone sensing
Creators: name: ABDUL HAROON RASHID, Syed Sulthan Alaudeen
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340108
subject: Sensor technology (incl. chemical aspects)
classificationCode: 401605
subject: Functional materials
classificationCode: 510204
subject: Photonics, optoelectronics and optical communications

description: Metal-oxide based sensitive layers are promising materials for acetone sensing applications. However, the major drawbacks of metal-oxide based sensitive layers are their lack of sensitivity and selectivity at low operating temperatures. This research centres on the investigation of tailored photoactive ZnO and TiO2 based nanoarray (NA) structures used in amperometric gas sensor (AGS) for their sensing capability towards low concentrations of acetone (&lt;100 ppm). The sensitive layers chosen for this study are based on ZnO and TiO2 composites such as ZnO-TiO2 core-shell, ZnTiO3 and Zn2TiO4 that were synthesised to form highly controlled nanostructured thin films. The composites were obtained by coating and annealing of TiO2 films of different thickness over well formed ZnO NAs substrates. The coating of TiO2 over the ZnO NAs in combination with controlled annealing resulted in diffusion of Ti4+ and Zn2+ ions within the resulting sensitive layer. The formed sensitive layers (ZnO-TiO2 core-shells) and zinc titanate composite (ZnTiO3 and Zn2TiO4) NAs are shown to exhibit increased sensitivity in comparison to the pristine ZnO NAs substrate. The gas sensing performance of the developed sensitive layers has been investigated towards low concentrations of acetone (1.2-12.5 ppm) at operating temperatures between 45-350 °C under the application of different potential biases ranging 0.1 to 9 V DC and UV photoexcitation conditions. The photoexcitation of the sensitive layers has been shown to increase the electron carrier density of the layer. This process is hypothesized to facilitate the formation of oxygen ions on the surface of the sensor, which then results in a significant increase in sensor response magnitude in comparison to when used under no photoexcitation conditions. There are two major innovations that are achieved in this work. The first regards the materials development, and centres on the synthesis of various ZnO-TiO2 based composites that form the highly controlled NAs structured sensitive layers. The second relates to the gas sensing approach which involved testing the developed sensors in amperometric mode by applying a fix bias potential (between 0.1 and 9V DC) while illuminating the sensitive layer surface with controlled intensity of UV light at controlled operating temperature. This enabled the surface reaction kinetics to be controlled and helped identify the conditions that optimised the sensor performance towards the target acetone concentrations used in this study. The light-assisted amperometric based acetone gas sensing approach has significantly improved sensitivity, repeatability limit of detection (LoD) and the adsorption/desorption characteristics of the sensor, whilst also improving the signal to noise ratio of the sensor baseline signal. The developed ZnO and TiO2 based nanoarrays sensors were observed to accurately sense low concentrations of acetone down to 1.2 ppm which is essential for applications such breath analysis for non-invasive diagnosis as well as process control and environmental monitoring applications. Among the materials tested in this project, the developed Zn2TiO4 NAs exhibited a LoD of 0.5 ppb and 4 ppb toward acetone when employed as the sensitive layer for UV-light assisted AGS at operating temperate of 350 and 45 °C, respectively.