Credit: © 2008 Wiley

Nanostructures in the form of wires and tubes have various applications in electronics, catalysis and medicine. These materials can be created using physical or chemical templating and laser-based processing. Researchers have now shown that ultralong nanorods can be formed by freezing and ageing an aqueous solution of nanoparticles1.

Using water-based methods, Sudipta Seal and colleagues at the University of Central Florida, Pacific Northwest National Laboratory and the UK Defence Academy made cerium oxide nanoparticles of around 3–5 nm in diameter by oxidizing Ce(III) precursor salts to Ce(IV). They then subjected the solution of nanoparticles to subzero temperatures and aged it over a number of weeks. Within one day, rods and octahedral superstructures were formed. Subsequent ageing for a week lengthened the nanorods, which grew into yet longer structures after two weeks. Similarly to the solidification of an alloy, it was suggested that as freezing occurs, nanoparticles are expelled into the water phase, saturating the unfrozen solution. As the ice front develops, capillary forces drive the nanoparticles from the saturated solution into channels and capillaries at the ice-water interface. Control experiments using unfrozen solutions indicated that the templating geometry of ice assisted the evolution of the nanorods. Furthermore, freezing by immersing the solution in liquid nitrogen did not form nanorods.

By varying the freezing rates, particle concentrations and ageing temperature to alter the ice structures, this approach may offer a green route to fabricate nanostructures.