VIDEO: Nanorods used to efficiently harvest water molecules from the air

06/24/2016 - 02:16

Frances White | Image: PNNL

After their nanorods were accidentally created when an experiment didn't go as planned, the researchers gave the microscopic, unplanned spawns of science a closer look.

Chemist Satish Nune was inspecting the solid, carbon-rich nanorods with a vapor analysis instrument when he noticed the nanorods mysteriously lost weight as humidity increased. Thinking the instrument had malfunctioned, Nune and his colleagues moved on to another tool, a high-powered microscope.


Ref: Anomalous water expulsion from carbon-based rods at high humidity. Nature Nanotechnology (13 June 2016) | DOI: 10.1038/nnano.2016.91


Three water adsorption–desorption mechanisms are common in inorganic materials: chemisorption, which can lead to the modification of the first coordination sphere; simple adsorption, which is reversible; and condensation, which is irreversible. Regardless of the sorption mechanism, all known materials exhibit an isotherm in which the quantity of water adsorbed increases with an increase in relative humidity. Here, we show that carbon-based rods can adsorb water at low humidity and spontaneously expel about half of the adsorbed water when the relative humidity exceeds a 50–80% threshold. The water expulsion is reversible, and is attributed to the interfacial forces between the confined rod surfaces. At wide rod spacings, a monolayer of water can form on the surface of the carbon-based rods, which subsequently leads to condensation in the confined space between adjacent rods. As the relative humidity increases, adjacent rods (confining surfaces) in the bundles are drawn closer together via capillary forces. At high relative humidity, and once the size of the confining surfaces has decreased to a critical length, a surface-induced evaporation phenomenon known as solvent cavitation occurs and water that had condensed inside the confined area is released as a vapour.