Surface acoustic wave based sensors for selective detection of low concentration elemental mercury vapour
Creators: name: Kabir, K M Mohibul
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340108
subject: Sensor technology (incl. chemical aspects)
classificationCode: 340604
subject: Electrochemistry
classificationCode: 400908
subject: Microelectronics
classificationCode: 401807
subject: Nanomaterials

description: Due to their highly toxic nature, mercury (Hg) has lots of adverse effects on human health and the environment. It is of no surprise that Hg emissions are considered as a major global concern and governing bodies around the world are now introducing stringent rules in order to reduce emissions from major anthropogenic sources. Efficient detection of elemental mercury (Hg0) vapour is of particular interest as it represents 64-90% of total mercury emissions as well as being the main source of other, more toxic forms that end up in the environment and food chain. The detection of Hg0 is also the major step in evaluating the potency of any implemented mercury removal technology within an industrial process. In this context, the major aim of this PhD project was to develop and investigate surface acoustic wave (SAW) based Hg0 vapour sensors that can detect Hg0 vapour concentrations below 400 ppbv (~3.6 mg/m3) in the presence of co-existing interfering gases (i.e. volatile organic compounds, humidity etc.) that are commonly found in many industrial environments and processes.<br><br>In the journey to develop such sensor, a critical literature review revealed that there were several major research questions and hence knowledge gap that needed to be fulfilled prior to successfully developing low concentration SAW based Hg0 vapour sensors. Upon attempting to address these challenges, SAW devices with different structural designs and sensitive materials were investigated for selective Hg0 vapour sensing where either interdigitated transducers (IDTs) or a dedicated sensitive layer were employed as the sensing element. These sensing elements were based on thin films of gold (Au), silver (Ag) or Ni-Au alloy nanostructures thereby allowed to determine the effect of SAW design and material type on Hg0 vapour sensing performance. The developed SAW sensors were all tested toward different concentrations of Hg0 vapour (24 to 365 ppbv) at various operating temperatures, ranging from 35 to 105°C depending on the sensor design and material. The data from each developed sensor was analysed in order to study the effect of operating temperature on each sensor’s performance in terms of response magnitudes, limit of detection, recovery efficiency, response time, sorption/desorption rates, calibration curve trends, sensitivity and selectivity, which are all important when employing such devices in real-world industrial conditions. The interfering gas species selected for different selectivity tests were chosen to be ammonia, acetaldehyde, ethyl mercaptan, dimethyl disulphide, methyl ethyl ketone and humidity, which are known to be commonly available at industrial processes such as Alumina refinery and mining industries. To obtain an in-depth fundamental insight into the Hg0 sorption characteristics on the sensor sensitive layers, finite element method (FEM) simulations were employed, which confirmed some of the assumptions that were made from the experimental data regarding the sorption and diffusion behaviour of Hg0 atoms on sensing surface.<br><br>Analysis of Hg0 vapour sensing data showed that the performance of the sensor depended heavily on the combination of structural design of the SAW device and the sensing materials employed. For instance, a lower LoD and higher Hg0 sorption capacity was achieved by employing Ag as opposed to Au as the IDT electrodes. When comparing two different SAW designs have the same sensing element, it was found that the IDT sensing element based design showed relatively lower LoD at low operating temperatures (i.e. 35°C) while the opposite trend was observed at higher operating temperature (i.e. 75°C). It was also found that the sensitivity of a SAW based Hg0 vapour sensor could be tailored by controlling the growth of Ni-Au alloy nanostructures on the SAW sensing surface, which can be done simply by changing the deposition parameters. The developed Ni-Au alloy based sensors showed faster response time than the Au-electrode based sensor while showing a lower LoD at elevated operating temperatures. Overall, the potential of SAW devices as selective Hg0 vapour sensor was extensively tested and devices’ feasibility for industrial application was analysed in detail.