Photocatalytically decorated Au-nanoclusters TiO2 nanofibers for elemental mercury vapor detection

Photocatalytically decorated Au-nanoclusters TiO2 nanofibers for elemental mercury vapor detection A. Macagnano1, E. Zampetti1, A. Bearzotti1, N. Pirrone1, G. Esposito1, F. De Cesare1,2 1. IIA-CNR, Monterorotondo (RM), Italy 2. DIBAF-University of Tuscia, Viterbo, Italy Summary Electrospun nanofibrous nets of titania (anatase) were deposited on intergitated micro-resistors and photocatalytically decorated with gold nanoclusters (AuNCs) upon UV irradiation. The resulting devices appeared to be conductive even at room temperature (Ohmic behavior), and were investigated as potential chemosensors capable of entrapping and detecting elemental mercury vapors in the atmosphere. The strong affinity and specificity of Au for Hg0 conferred to AuNCs a key role in mercury sensing, by inducing a decrease in current upon mercury adsorption. Sensors conductivity appeared not to be dependent on humidity. The resulting chemosensors are expected to be inexpensive, very stable (due to the peculiar structure), and requiring low power, low maintenance and simple equipment to work. Motivation and results Mercury is a well-known neurotoxic and persistent worldwide pollutant. Since 2010 a European project called Global Mercury Observation System (GMOS) has been involved in creating an international network capable of providing accurate measurements of Hg on a global scale (www.gmos.eu). Presently, air monitors are highly sensitive and capable of detecting the global mercury background, but are complex, costly and high-maintenance (requiring more than 40 monitoring sites and skilled operators). In the present study, hybrid nanofibrous electrospun1 layers of TiO2 coating Pt microresistors were suitably designed in order to create nanostructured 3D-frameworks, potentially capable of adsorbing and revealing elemental mercury vapors in the atmosphere. Specifically, titania fibers were decorated with gold nanoclusters (AuNCs) (Fig. 1,a) induced to grow selectively upon photocatalytic reduction of HAuCl4 caused by UV light irradiation2. The morphology and size of gold nanoclusters, as well as the coverage of fibers, depended on UV irradiation exposure (Fig.1,b-c). The sensors showing Ohmic behavior in air at room temperature (Fig. 2) were here investigated. The strong affinity and specificity of Au for Hg0 explain the sensitive responsiveness of AuNCs on the nanofibrous scaffolds to mercury3, highlighted by the current changes of the sensors upon Hg adsorption. In detail, sensors exposed to mercury vapours, pointed out a decrease in current, and the slope of the electrical signals seemed to be dependent on analyte concentration. When sensors were exposed overnight to high concentration of Hg (PV=1.6o10-3 mbar), their resistance changed by 30% (Fig. 3). In order to test the responsiveness of the device to lower Hg concentration, a dynamic permeation tube was set up to dilute the mercury-saturated gas to the concentration range of interest, by using a mass flow controller system (4-Channel-MKS 247). The final Hg0 concentration (measured by Tekran®2537A) was obtained by tuning the oven temperature and the dilution flow. Relative humidity did not affect the electrical parameters of the sensors. Preliminary results showed that the exposure of sensors to 7.5ng/l of Hg resulted in a decrease in current, with a rate of ~109 nA/min (+2V). By modifying Hg concentration, the adsorption rate also changed. Quick thermal shots could restore sensors, due to the high porosity of the layer and the extremely reduced size of the gold nanoclusters.