Nano-engineered surfaces developed for mercury sensing
Creators: name: Sabri, Ylias
Publisher: name: RMIT University
Year: 2024

description: Mercury is highly toxic but often neglected element that is readily emitted into the environment via a number of industrial processes. In Australia, coal-fired power plants and the alumina industry are the two largest emitters of mercury. In order to maintain the alumina industry’s commitment to reduce the environmental impact of its processes and remain economically sustainable, innovative technologies are required that can monitor mercury concentrations within its processes. The aim of this research project was to develop robust quartz crystal microbalance (QCM) based sensors for measuring Hg vapour levels in challenging industrial environments, such as those found in the alumina industry (i.e. Hg concentrations of 1-40 mg/m3 at 20-90°C).<br><br>In order to gain a deeper understanding of Hg vapour interactions with the surface of the QCM electrodes, parameters such as Hg sorption and desorption rates, sticking probability and Hg diffusion were studied in detail. When Hg diffusion in ultra-thin films of Au was studied, it was observed that Hg accumulation occurred at the interface between the Au sensitive and SiO2 layers. These observations lead to the exploration of two different electrochemical methods for the direct formation of Hg selective and sensitive nanostructured Au films directly on QCM crystals. In the first case, galvanic replacement (GR) reactions were employed to form Ni-Au hybrid nanoclusters. This method resulted in Hg sensors with 93-100% recovery, excellent selectivity towards Hg in the presence of ammonia and humidity and ~27% higher sensitivity than the Au control QCM. The second case involved forming highly oriented and ornate Au nanostructures (nanospikes) with controlled crystallographic facets onto the QCM electrode by a single step electrodeposition method. These sensors were tested towards Hg vapour in the presence of ammonia and humidity at ~90°C for a 50 day period in a specially designed and developed 8-channel computer controlled gas calibration system. The testing sequences were made to simulate some of the conditions found in Hg-emitting industries. The nanospike QCM showed high selectivity, recovery and around 4.7 times higher sensitivity than the Au control QCM, with low degradation in response magnitude over the long testing period. <br><br>The high sensitivity of the nanospikes was found to be not only due to high surface area but also due to the increased number of surface defect sites created during the electrodeposition step. Due to its excellent performance, a nanospike QCM was then tested towards Hg in the presence of several volatile organic compounds (VOCs) that either had high affinity towards Au or was present in alumina effluent streams. The nanospikes based QCM was tested against VOCs such as acetone, dimethyl disulphide, methylethyl ketone, ethyl mercaptan, acetaldehyde as well as ammonia and humidity. The nanospike QCM was observed to maintain high selectivity and sensitivity towards Hg vapour when compared to Au control vi QCM. The success of the data presented in this thesis has resulted in a PCT patent of the developed nanospikes and is due to undergo preliminary testing at industry partners’ sites. If successful, the developed sensor will assist industries in complying with mercury emission targets and would be a significant technological breakthrough with potential for many other applications in pollution control.<br>