Development and fabrication of room temperature mixed metal oxide based ozone sensing elements for health and safety applications

Atmospheric pollution is defined as a status containing gases, offensive odours and particles that are harmful to humans, animals, vegetation or living environments above the regulation limits specified by regulatory bodies in the specific areas. Atmospheric pollution has become particularly serious since the Industrial Revolution. Furthermore due to the recent dramatic growth in population and industrial development, coupled with an intensified usage of fossil fuels, the natural atmospheric environment has become severely polluted and is rapidly deteriorating. As the level of public concern related to living and working in a healthy environment has now increased, the demand for monitoring and controlling the atmospheric environment in the house and workplace has also increased. As a result, intensive research efforts have been made in various fields in an attempt to resolve such environmental problems or dangers. Indoor sources of ozone are more common than they were in the past due to the introduction of electronic equipment. Some of the most significant ozone generating equipment includes photocopiers, laser printers, air purifiers, ozone generators and electrostatic precipitators. Exposure to ozone at extremely low concentrations (Table 2.2 – World Health Organisation, 43-85ppb) constitutes a human health hazard. Exposure is known to decrease the short-term lung function and adversely affect the respiratory system. Prolonged exposure to relatively low levels has been linked to increases in morbidity and mortality rates. This work undertakes to develop easily producible, cost effective and novel metal oxide based gas sensors capable of operating at room temperature for the detection of environmentally relevant ozone. Fabrication techniques of novel metal oxide mixtures are explored and these sensing layers are further developed/optimized and analysed. Room temperature ozone sensors have been shown to be sensitive to low ppb levels of ozone. Operation of these sensor types at room temperature has the added advantages of reduced fabrication costs, reduced operating costs, low power consumption and ease of implementation into other portable/handheld devices. In this work, mixed In2O3, ZnO, SnO2 and sole NbO2 oxide sensing layers were fabricated and studied. Optimization of the composition and concentration of the mixtures was undertaken in an attempt to discover the best sensor performance. It was found that mixing of the above oxide materials and simultaneous evaporation from a molybdenum boat yielded workable room temperature ozone sensors. Optimization of the fabrication process was also studied in detail and the effect of changing the various fabrication parameters was documented and analysed. It is shown that decreasing the sensing layer thickness of these devices, while increasing the deposition rate of the material from the molybdenum boat yields to a significant improvement in overall sensor performance. To understand the nature of the sensing films, the surfaces of the sensing layers were examined via SEM analysis. For these SEM images it can be seen that higher deposition rates result in more porous sensing layers. XPS analysis of the sensor samples (wide and narrow scans) was carried out in an attempt to better understand the increases in sensor performance. From these results it was seen that there are slight shifts in the binding energies of the metal oxide components which is known to have an effect on the sensitivity of the devices. Also of particular interest from these results is the presence and shape of the O 1s peaks and the corresponding performance of the sensing layer with respect to oxygen vacancies present. This work has led to the successful use of the vacuum thermal evaporation process to fabricate room temperature ozone sensing elements based on a mixture of In2O3, ZnO and SnO2 as well as an NbO2 based ozone sensor, both of which are capable of detecting ozone levels as low as 50ppb.