Formation of Mercury Droplets at Ambient Conditions through the Interaction of Hg(II) with Graphene Quantum Dots

Abstract

Unlike other metals, Hg forms droplets at ambient conditions when a Hg(II) salt interacts with hydroxyl-enriched graphene quantum dots (HEGQDs). The hydroxylation of GQD surface is evident from FT-IR, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) techniques. The scanning electron microscopy images of Hg(II)-HEGQDs incubated for 0, 1, 24, and 168 h show Hg droplets with the size of 0.1, 0.3, 0.8, and 2 μm, respectively. The XPS studies confirm the presence of Hg(0) and also reveal a noticeable decline in the composition percentage of C-O, whereas a marked increase is observed in the C═O composition percentage. The pathway for the formation of droplets induces immediate reduction of Hg(II) to Hg(0) by both hydroxyl groups and π electron cloud present on the surface of HEGQDs, followed by coalescence. The formed Hg(0) is then strongly adsorbed on the hollow sites of graphene and acts as a nucleation site for the growth of droplets. The kinetics of the reaction obeys LaMer Burst nucleation followed by coalescent growth in addition to autocatalytic reduction and finally follows the Oswald ripening mechanism. The internal pressure of Hg droplets gradually decreases as the radius of the drop increases over the incubation time and liquid-rhombohedral transformation is likely to take place at a radius of 0.8 nm.