Formation of size-selected Ni/NiO core-shell particles by pulsed laser ablation

This paper presents the method for fabricating size-selected nickel nanoparticles (diameter: 5-20 nm) coated with nickel oxide shells (thickness: about 2 nm). The core-shell particles were synthesized by a series of sequential gas-phase processes including pulsed laser ablation, aerosol postannealing, size-classification, compulsory oxidation, and aerosol-jet deposition. The change in the size distribution of the generated particles due to postannealing was measured by a low-pressure differential mobility analyzer (LP-DMA) coupled with a Faraday cup electrometer. The peak size decreased as the postannealing temperature rose, indicating that a sintering and restructuring of the agglomerates was taking place. In the high-resolution TEM (HRTEM) observation, the Ni particles of less than 15 nm in diameter were found to be spherical and single crystal after the annealing at 1073 K. The 20 nm particles were also spherical, but they remained polycrystalline due to insufficient annealing treatment required for recrystallization. The change in the surface morphology due to the compulsory oxidation was analyzed by the HRTEM and electron diffraction. Finally, monodispersed (geometric standard deviation < 1.2) Ni/NiO core-shell particles with oxidized surface layers of 2 nm and particle diameters ranging from 5 to 20 nm were successfully fabricated.