Mercury diffusion in gold and silver thin film electrodes on quartz crystal microbalance sensors

Diffusion behavior of mercury into both gold and silver electrodes of polished and roughened quartz crystal microbalances (QCMs) is presented. Several QCM devices were exposed to mercury vapor for 8 h and allowed to desorb for 5 h under controlled nitrogen atmosphere. The process was repeated for different Hg concentrations of 1.02, 1.87 and 3.65 mg/m3 at an elevated temperature of 40 °C. The chemical composition and surface morphology of each QCM surface was characterized by Inductively Coupled Plasma Mass Spectroscopy (ICP-MS), Secondary Ion Mass Spectroscopy (SIMS), Atomic Force Microscope (AFM) and Field Emission Secondary Electron Microscope (FE-SEM). The Ag electrodes were found to contain up to 16% more adsorbed/amalgamated Hg by mass than the Au samples. It was found that the ratio of amalgamated to adsorbed Hg is less for Au than Ag. SIMS analysis confirmed high Hg diffusion through rough substrates, 40 days after Hg exposure. In situ sticking probability of the tested mercury vapor concentrations to Au and Ag surfaces at 40 °C was found to drop at quicker rates than the reported Hg-Au and Hg-Ag room temperature values. Overall, in the context of Hg vapor phase gas sensing applications, the rougher gold substrate was found to outperform the other samples due to its superior adsorption/desorption properties.