1.Introduction In the past decade, numerous efforts have been taken to develop one-dimensional nanomaterials with specific morphologies such as nanowires and/or NAs because of their large surface to volume ratio. These morphologies also offer a possibility for surface modification so their versatile material properties can be used for different applications. ZnO NAs has been widely used as a sensing layer but the major drawbacks of metal-oxide based sensitive layers are their lack of sensitivity at low operating temperatures. In this work, the ZnO NAs are coated with a thin TiO2 layer to form ZnO-TiO2 composite materials. The combination of ZnO and TiO2 has attained significant attention because of the synergistic advantages these materials could contribute [1]. Several studies have used ZnO-TiO2 composites as a gas sensing layer, however there is a void in the literature, especially in NAs structures for acetone sensing. Hence, this work centres on the investigation of tailored ZnO and TiO2 composites in NAs structures under light illumination for their sensing capability towards low concentrations of acetone (<100 ppm). 2.Experimental In this work ZnO-TiO2 based composite NAs were synthesised by coating a thin layer of (~50 nm) TiO2 using low-temperature CVD on the ZnO NAs grown by hydrothermal synthesis. The composition of TiO2 and ZnO added, the annealing temperature and time plays a crucial role in determining the crystal phase of the material [2]. The TiO2 coated ZnO NAs were annealed at 550 °C (1h, air) to form ZnO-TiO2 core-shell NAs and further annealed at 650 °C (1h, air) to form a mixed metal oxide, hence ZnTiO3 NAs. The surface morphology of the sensitive layers developed were studied using FEI Verios 460L SEM instrument. The acetone vapor sensing event included acetone exposure and recovery (under dry air) steps, each set at 5 minutes. All sensors developed were tested toward different acetone concentrations (ranging from 1.2 to 12.5 ppm). The acetone sensing performance was tested under operating temperatures 45 - 350 °C by applying a potential bias of 6 V DC. A UV light source with a wavelength of 365 nm (maximum intensity up to 2024 µW·cm-2) was used for light-assisted gas sensing. 3.Results and discussion The synthesis procedure schematic and the corresponding SEM images are presented in Figure. 1a-e. The ZnO NAs had an average length and diameter of 1.5 µm and ~30 nm, respectively. The images (Figure. 1d-e) suggest that the thicknesses of the NAs increased significantly after the ~50 nm TiO2 coating. The gas sensing performance of the developed sensors showed a poor dynamic range (Figure. 2a) at 85 °C in dark conditions. When illuminated with light, the sensors showed enhanced response magnitudes and dynamic range (Figure. 2b and 2c). The photo excitation of the sensitive layers has been shown to increase the electron carrier density of the layer, which in turn is hypothesized to facilitate the formation of oxygen ions on the surface of the sensor, which results in a significant increase in sensor response magnitude [3]. In comparison to the pristine ZnO control sensor, the ZnO-TiO2 composite NAs had much higher response magnitude. The annealing of the ZnO NAs coated with TiO2 initiates the diffusion of Ti from the titania shell to the zinc core and vice versa. The Zn2+ ions get substituted with Ti4+ ions in the core thereby the ZnO core becomes electron rich and also increases the affinity to adsorb more oxygen on the surface. The possible reason for the ZnO-TiO2 core-shell NAs to have higher response than the ZnO NAs could be the reactivity of TiO2 in tailored NAs morphology with high surface to volume ratio. Further, the formation of ZnTiO3 NAs introduces structural defects and oxygen deficiencies which results in higher oxygen adsorption on their surface in comparison to pure ZnO sensors. The LoD of the ZnO NAs, ZnO-TiO2 core-shell NAs and ZnTiO3 NAs sensors calculated to be 90, 70, 30 ppb respectively and the sensitivity, adsorption (t90-ads) and desorption (t90-des) time are presented in Table. 1. 4.Acknowledgements The support of the Australian Research Council Discovery Project DP150101939 is acknowledged. All authors acknowledge the RMIT Microscopy and Microanalysis Facility (RMMF) for their scientific and technical assistance and for providing access to their comprehensive infrastructure and research facilities.
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Jan 31, 2022
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