Single-crystal Co nanoparticles in hcp and fcc allotropes synthesized by CW laser ablation

ABSTRACT The synthesis of nanocrystals with controlled size distribution is of key importance, as the nanoscale dimensions strongly influence their properties. Single-crystal pure Cobalt (Co) nanoparticles with a median diameter of about 3.9 nm have been synthesized via a novel green route: low-power continuous-wave (CW) laser ablation in de-ionized water. De-ionized water played a key role in suppressing anisotropic growth, yielding both spherical and aspherical cobalt nanoparticles. Characterization by Transmission Electron Microscopy (TEM), High-Resolution TEM (HR-TEM), and X-ray Diffraction (XRD) revealed the nanocrystal phase transformation, interatomic spacing, crystal structure, and chemical composition of the as-prepared Co nanoparticles. Both hcp and fcc allotropic phases of cobalt nanoparticles are observed. These allotropic phase transformations are attributed to both the lower surface free energy of the fcc-Co phase in the observed size regime and to the rapid solidification of liquid nanodroplets during CW laser processing at low irradiation power. Thus, low-power CW laser ablation methods provide a single-step, energy-efficient, non-catalyst-based route for the production of stable, pure, well-dispersed, and crystalline nanoparticles without any post-synthesis treatment. These nanostructures can be manipulated further by altering the synthesis parameters.