Gas sensing performance enhancement: Determining the role of active sites through colloidal lithography

Abstract

Using colloidal lithography, gold honeycomb (AuHC) monolayer crystals with various degree of porosity were formed on quartz crystal microbalance (QCM) electrodes thus enabling the experimental determination of the important role that surface parameters (pore size, surface area etc.) play in the sensing of gaseous species such as mercury vapor. The pore size was controlled through altering the O2 plasma etching periods of the close-packed monodispersed polystyrene colloidal crystal templates. It was found that the reduced pore size (or increased surface area) of AuHC structures enhanced the sensitivity of the QCMs toward mercury vapor adsorption, however the enhancement could not be correlated to the surface area increase alone. For example, the QCM containing AuHC structures which had been etched for 12 min (AuHC12 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°) vapor 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 AuHC12 min QCM showing 7.4 times higher response magnitude toward Hg° vapor over the AuHC0 min based QCM when operated at 30 °C. Furthermore, the selectivity of the AuHC12 min based QCM 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.