Liquid phase temperature salts dissolve metallic catalysts like Fe, Co, or Ni, and “wash” them away.
John H. Glenn Research Center, Cleveland, Ohio
Physical and chemical properties of nanomaterials are known to be significantly different from those having larger crystallites (i.e. bigger than nano), but with the same chemical compositions. Optimal uses of these new nanomaterial properties will likely result in engineering materials that are better than what is available today. Before this can happen, characterization of the physical and chemical properties of nanomaterials is needed.
The purpose of this innovation is to purify the as-synthesized nanomaterials; that is, to remove catalysts and/or excess reactants from nanomaterials newly produced from chemical reactions. The focus is on removing impurities that are hard to remove by other methods, and with minimal damage to the nanomaterials.
It is observed that ferric chloride (FeCl3) at or near its liquid phase temperature can be a solvent that reaches internal surfaces of the nanomaterials, and can dissolve the impurities embedded inside of them. The FeCl3-impurity solution (or mixture) can then be rinsed away by water or acids. This process removes residue impurities left in the nanomaterials after the as-synthesized nanomaterials were treated by more conventional cleansing processes.
An as-produced, raw boron nitride nanotube (BNNT) sample was treated sequentially with 60 ºC HCl for 13.5 hours to remove the impurities external to BNNT; exposed to 700 ºC air for 15 minutes to effectively oxidize alloys of Fe, B, and P to a mostly acid-soluble compound; exposed to room temperature HCl for 39.5 hours to dissolve the oxidized compounds; exposed to 1150 ºC N2 for 30 minutes to anneal the BNNT structure and to drive phosphorus out of the boron nitride structure; exposed to 310 ºC FeCl3 for 13 hours to remove non-boron impurities such as P and Fe; and then rinsed with deionized water, treated with room-temperature HCl for 3 hours, and rinsed with deionized water again to wash away the chemicals dissolved in FeCl3.
Pure FeCl3 melts at 306 ºC and boils at 315 ºC, but this method was found to be effective when FeCl3 was in a wider temperature range of 250 ºC to 350 ºC. This method can be applied as long as the nanomaterials can be wetted by FeCl3, and the impurities can be dissolved by molten FeCl3 or reacted to FeCl3 to form a dissolvable or gaseous product. It does not need to be restricted to boron nitride nanomaterials and Fe or P impurities. An example is carbon nanotubes containing Co or Ni.