Studying the effect of dealloying Cu-Au nanostructures on their mercury sensing performance

In this study we demonstrate that the dealloying of Cu-Au nanostructures electrochemically deposited on the electrodes of a quartz crystal microbalance (QCM) can significantly increase its sensing performance (sensitivity, reversibility) toward low concentrations of elemental mercury (Hg-0) vapor. The Cu-Au alloy nanostructures were electrodeposited on the Au electrodes of the QCM device for periods of 2, 4 and 8 min from an electrolyte containing equimolar concentrations of CuSO4 and HAuCl4 salts. When tested with Hg-0 vapor concentrations ranging from 24 to 365 ppb(v), it was found that the 4min Cu-Au alloy had up to 10.4 times higher response magnitude than a control Au electrode-based QCM sensor. Furthermore, when dealloying experiments were conducted, it was found that electrochemical dealloying periods of 4 and 8 min significantly increased the reversibility of the 4 min Cu-Au alloy nanostructure based sensor. For instance, the structures made by a 4min-8 min alloy-dealloy periods showed up to 5.6 times higher response magnitude toward different concentrations of Hg-0 vapor when compared to the control Au thin-film (Au-ctrl) based sensor. In addition, the same sensors exhibited an average reversibility of 84.4% following Hg-0 vapor exposure, which is high when compared to the average reversibility of 63.3 and 44.4% observed from the Au-ctrl and Cu-Au (4 min alloy) based sensors, respectively. Surface characterization and analysis indicated that the enhanced sensitivity of the Cu-Au based sensors was not just due to higher surface area but also due to the surface defects that are formed on the surface during electrochemical treatments. Overall, the results indicate that the low concentrations Hg-0 vapor sensing performance of a QCM device can be tuned by controlling the alloy and dealloying conditions of the Cu-Au system directly on the electrode surface. The sensor with optimum alloying and dealloying time (4min-8 min) showed good repeatability and selectivity with the sensor showing a coefficient of variance of 5.45% when exposed with the mixture of common industrial interfering gas species such as ammonia, acetaldehyde and ethyl mercaptan. Furthermore, the sensor was also showed a low coefficient of variance of 3% when exposed without and with the presence of water vapor, indicating the sensor's high repeatability and selectivity even in humid environments. (C) 2017 Elsevier B.V. All rights reserved.