Pure face-centered-cubic (fcc) and hexagonal-close-packed (hcp) nickel phases obtained in air atmosphere sol-gel process and fcc nickel phase obtained in N2 protected sol-gel process

Air atmosphere sol-gel process is often applied to prepare metallic oxide materials. Here we show that metallic nickel phase can also be obtained by citric acid (CA) based air atmosphere sol-gel process in appropriate procedure. Pure fcc or hcp phases of nickel can be obtained in air atmosphere by using CA, nickel (II) acetylacetone, organic solvents and organic surfactants such as oleylamine (OAM), hexadecylamine. However, only NiO can be prepared by aqueous CA based sol-gel process. Oleic acid and OAM can also be used in organic solution mediated sol-gel process to prepare nickel nanoparticles in N-2 atmosphere with pure fcc phase at appropriate calcination temperature when 1-hexyl alcohol has been used as solvent. Impurities, such as hcp Ni or Ni3N phase, do not occur in this experimental condition. Our results provide a new and facile way in preparation of metallic nanoparticles.