Investigation of Hg sorption and diffusion behavior on ultra-thin films of gold using QCM response analysis and SIMS depth profiling

Abstract

Using the quartz crystal microbalance (QCM) technique, we demonstrate that the contribution of Hg adsorption and absorption on the sensor response profile can be distinguished by studying the dynamic response curve of QCM based Hg vapor sensors that employ an ultra-thin film of Au in the range of 10 to 40 nm thickness as the sensitive layer. The response magnitudes of the QCMs were extrapolated to zero thickness (ETZT) in an attempt to determine the contribution of adsorbed Hg on the sensor response magnitude and response profile. In general, the ratio of adsorbed to absorbed Hg on Au films is found to decrease with increased Hg vapor concentration. Furthermore, the same ratio was observed to decrease with increasing Au film thickness. The 10 nm and 40 nm Au films for example were found to contain adsorbed Hg content of 43.8% and 16.4%, respectively, with the balance attributed to absorption/amalgamation, when exposed to Hg vapor concentration of 10.55 mg m À3 for a period of 14 hours and an operating temperature of 28 C. In addition, the QCMs were characterized using secondary ion mass spectroscopy depth profiling in order to study the diffusion behaviour of Hg in the Au surfaces. It is deduced that in order to reduce Hg accumulation in Au thin films, a non-continuous type film (similar to the 10 nm ultra-thin Au sensitive layer morphology) would be more functional as a Hg sensitive layer where quick absorption and desorption processes are required.

Mercury Sorption and Desorption on Gold: A Comparative Analysis of Surface Acoustic Wave and Quartz Crystal Microbalance-Based Sensors

“…This is much deeper than the previously reported study in the literature 37 indicating that Hg can diffuse to great lengths into Au films with long exposure to Hg 0 vapor. 38 The AFM image and the corresponding height profile show the formation of even electrodes with 100 nm thickness on the SAW device as shown in Figure 8d, corresponding to the Au layer and underlying Ni and Ti adhesion layers on the substrate. 4.6.…”

Development and comparative investigation of Ag-sensitive layer based SAW and QCM sensors for mercury sensing applications

“…37 It should be noted that a minimum thickness of 110 nm for the QCM electrodes (10 nm Ti adhesion layer + 100 nm Ti film) was necessary in order to achieve a better quality factor. 38 However, in case of the SAW sensor the sensitive layer thickness was kept at 60 nm (10 nm Ti adhesion layer + 50 nm Au/Ag film) in order to keep the sensor insertion loss at an acceptable level. The thicker sensitive layer in a SAW electrode would cause mass loading and thereby could further increase the insertion losses of the fabricated SAW device.…”

Silver/gold core/shell nanowire monolayer on a QCM microsensor for enhanced mercury detection

“…The QCMs tested in this study were ensured to have high quality (Q-factors > 3000) as was required by the in-house built calibration system, calculated using the method reported previously. 34 The developed QCM sensors' performance were tested toward elemental mercury vapor sensing in a dynamic range of Hg 0 vapor concentrations at two operating temperatures of 30 C and 75 C AE 1 C. The operating temperatures were selected based on their relevance to room temperature sensing and on-eld sampling conditions used by industry to sample Hg 0 vapor. 35 In addition, the selectivity of the sensors toward Hg 0 vapor were tested by exposing the sensors toward Hg 0 vapor in the presence of industrial simulated effluent gases containing ammonia, (NH 3 , 384 ppm), ethyl mercaptan (Ethyl-M, 2.61 ppm), acetaldehyde (Ac-ald, 304 ppm), dimethyl disulde (DMDS, 5.01 ppm), methyl ethyl ketone (MEK, 40.1 ppm) and high levels of humidity (H 2 O, 27.2 g m À3 ).…”