Synthesis and chemical reactivity of polyol prepared monodisperse nickel powders

Monodisperse 1350 Angstrom Ni powders are prepared according to the polyol process using polyvinylpyrrolidone (PVP) as a protective agent. Smaller size monodisperse Ni powders (300 Angstrom) have presently been made from NI(OH)(2) in ethylene glycol (EG) and PVP using Pd or Pt as nucleating agents. Thermogravimetric Analysis (TGA) and Temperature Programmed Desorption (TPD) measurements in air showed that ethylene glycol chemisorbed on the Ni surface protects the fine particles from air oxidation up to 180 degrees C. The oxidation peak temperature is reduced from 650 degrees C for Ni of 1 to 2 mu m to 260 degrees C for Ni of 300 Angstrom. The unseeded pure Ni powder undergoes stoichiometric oxidation to become NiO with a 27% weight gain while the oxygen uptake is 18.6% for 300 Angstrom Ni particles seeded with 1% Pd. In that case, the formation of a new (Ni1-xPdxO1-y) type oxide crystallizing in a tetragonal structure similar to PdO is suggested. While the Ni particles of 1350 Angstrom mean size followed a parabolic oxidation law (% oxide formed(alpha)t(1/2)), 300 Angstrom Ni particles followed a cubic law (% oxide(alpha)t(1/3)). For Ni seeded powders with 1% Pt, the oxygen uptake is only 15.7%; the remaining Ni not undergoing oxidation is an Ni1-xPtx alloy phase. A model calculation shows that 300 Angstrom Ni particles grow around 70 Angstrom Pd or 60 Angstrom Pt nuclei and that the particles are covered by half an EG monolayer.