Temperature and ambient induced band structure changes in tin oxide to optimize nano-sensors for safety applications

Abstract

The increasing demand for efficient energy harvesting and sensing devices with low fabrication cost, has attracted a lot of scientific research effort the last ten years. In particular, the scientific community investigates new candidate materials suitable for devices, such as sensors and photovoltaics or clean energy applications such as hydrogen production. Over the years we observe that many methods are examined to improve the efficiency of the devices with the doping method being the most promising one. The ideal gas sensor is the one that has high response towards small gas quantities, has selectivity towards different gases and is stable. Combined with all the above, it is also crucial for the device to have low energy consumption and even work at room temperature. The aim of this dissertation is to investigate and fabricatereliable gas sensors with low energy consumption with the doping method. Herein, halogen doping is chosen as a low cost and appropriate method for enhancing and provide a deeper understanding on their atomic scale mechanisms with respect to their potential applications in photovoltaics, sensors and hydrogen production. To achieve this, density functional theory (DFT) calculations are used to examine the defect processes, the electronic structure and the optical properties for halogen doped SnO2. For further investigation towards better gas sensing devices, experimental characterization techniques have been used. Specifically, with the Ultraviolet-Visible spectroscopy (UV-Vis) and the Ultraviolet Photoelectron spectroscopy the optical properties and the bandgap changes are investigated in order to gain a better understanding of the changes of SnO2 upon halogen doping and how these bandstructure changes can be used for sensors, with the Fourier Transform infrared spectroscopy (FTIR) and the X-ray photoelectron spectroscopy (XPS) are used for identifying the composition of the materials as well as the percentage of halogen doping. Gas sensors are highly dependent to crystallite size and surface morphology and for that reason X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Atomic force microscopy (AFM) are essentials forhighlighting the changes due to halogen doping and how this changes are crucial for the improvement of gas sensors. The characteristics of halogen doped SnO2, except for gas sensors, were also suitable for other applications such as hydrogen production. Indeed, halogen doping improved the photocatalytic propertis of SnO2 which is regarded a “bad photocatalyst”. TiO2 is often regarded as an alternative for energy applications but still the as discussed it has many limitation due to its wide bandgap and reduced conductivity. The method of halogendoping is also examined for TiO2, were it is seen that the optical, electronic and photocatalytic properties are further enhanced. The presented results are expected to motivate further research and can also be used on photovoltaic and sensor device fabrication and to encourage the research for alternative dopants that can further improve the efficiency of SnO2 devices

Date of Award	Mar 2023
Original language	English
Awarding Institution	Coventry University
Supervisor	Stavros Christopoulos (Supervisor), Alexander Chroneos (Supervisor) & David Parfitt (Supervisor)