Formation of Salt Crystal Nano-Whiskers on Nanoporous Coatings

H. Zhang, L.F. Francis
University of Minnesota, US

Keywords: whisker, single crystal, nanowire, porous coating, KCl, NaCl

Summary:

Salt crystal nano-whiskers were grown from aqueous solution onto nanoporous silica coatings for the first time. Nanoporous silica coated glass substrate was partially immersed into an aqueous KCl solution and then kept in a controlled relative humidity chamber for whisker growth. Single crystal salt whisker morphologies, typically 2 - 50 µm in lateral dimension and up to ~3 cm in length, emerged from the coating surface. Whiskers are confirmed to grow by a base growth mechanism. Whisker specimens were also quickly frozen during growth and studied by Cryo-SEM, revealing supporting liquid layer at the whisker base. Sheet-like crystals also formed from whiskers that had fallen flat onto the nanoporous coating surface. A seeding technique, inspired by the sheet formation, was developed. Finely ground salt particles were distributed on the nanoporous coating before whisker growth to serve as seeds. By a careful choice of seed concentration and nanoporous coating structure, salt nanowhiskers with lateral dimension as small as 50 nm were obtained on the surface of the coating. This crystal growth method can also be applied to different materials, namely water soluble materials, and creates whisker crystals with controllable size and location on the nanoporous coating.