Highly selective and sensitive nano-engineered materials for detection of elemental mercury vapour

Anthropogenic release of heavy metal mercury (Hg0) vapour is of global concern due to its well-known bio-accumulative and neurotoxic properties. The detection of Hg0 is of upmost importance as it can transform to the relatively deadlier oxidised form of mercury once released into the environment and end up in the food chain. Therefore, due to these concerns, the aim of this PhD project was to develop rapid,  cost-effective and robust quartz crystal microbalance (QCM) based sensors that can selectively sense Hg0 vapour in the presence of interfering gases (i.e. volatile organic compounds, humidity etc.) that are found in challenging industrial process and the environment. In pursuance of gaining a deeper understanding and developing a more selective and sensitive nano-engineered material, several parameters such as surface structure, alloy content, surface topology and actives sites were investigated. Two different modifications such as galvanic replacement reaction and colloidal lithography were employed to provide fine control over surface chemistry and morphology for the direct formation of nanostructures on QCM crystals. The developed QCMs were investigated towards mercury by testing them towards different concentration Hg0 vapour at various operating temperature. The data of each sensor was studied to determine the effect of the operating temperature as it has a significant effect on several critical parameters (i.e. sensitivity, selectivity, recovery efficiency, limit of detection (LoD), repeatability, response time, adsorption/absorption/desorption rates etc.). The feasibility of the developed QCM based Hg0 sensors were exposed towards Hg0 vapour in the presence of industrially relevant gas species such as ammonia (NH3), acetaldehyde (MeCHO), ethyl mercaptan (EM), dimethyl disulphide (DMDS), methyl ethyl ketone (MEK) and humidity, all of which are commonly found to co-exist within many industrial processes such as the mining and alumina industries. Analysis of the Hg0 Sensing data showed that several parameters plays important part determining the performance of the sensing material. For instance, increased Au loading on the thin film surface does not increase mercury affinity of the surface; rather it was more important to produced small Au nanoparticles thereby increasing the surface volume ration for better sensitivity and response time. With the use of colloidal lithography, porous surface such as Inverted long-range crystals (i-LROCs) were create. It was determined that the reduced pore size (or increased surface area) of the i-LORCs structures enhanced the sensitivity of the QCMs toward mercury vapour adsorption, however the enhancement could not be correlated to the surface area increase alone. For example, the QCM containing gold honeycomb (Au i-LROCs) structures which had been etched for 12 min (Au i-LROC 12 min) had similar Au mass and surface area as the control QCM with its continuous thin film of Au (Au-Ctrl), yet exhibited around twice the response magnitude when exposed toward elemental mercury (Hg°) vapour at 30 °C. The increased response magnitude could only be explained by the increased number of available active sites undergoing Hg° sorption which are present around the edges of the honeycomb pores. The QCM data showed that the number density of these active sites increased with increasing plasma etching periods as evidenced by the Au i-LROC 12 min QCM showing 7.4 times higher response magnitude toward Hg° vapour over the Au i-LROC 0 min based QCM when operated at 30 °C. A combination of colloidal lithography and GR reaction can be used to produce homogenously decorated bi-metallic Pd-Au nanostructures which enhance the sensitivity and selectivity of the sensors. Furthermore, the selectivity was shown to be operating temperature dependent when undergoing cross-interference tests against common industrial gases such as humidity, ammonia, mercaptans, ketones and aldehydes, where improved selectivity was observed at elevated operating temperature of 75 °C.