In this paper, we present a simple and controllable preparation of face centered cubic (fcc) and hexagonal close-packed (hcp) Ni nanoparticles by a pulsed Nd:YAG laser ablation method in the following four solvents: deionized water, methanol, hexane, and acetonitrile. We generated Ni/NiO, fcc, and/or hcp Ni nanoparticles by primary ablation to a Ni plate submerged in various solvents, followed by secondary ablation to the colloidal solutions. Interestingly, the phases of Ni nanoparticles prepared via a pulsed laser ablation in liquid (PLAL) show a strong dependence on the solvents used in the ablation processes. Ni/NiO, pure fcc, and a mixture of fcc and hcp Ni nanoparticles were generated in DI water, methanol, and hexane or acetonitrile, respectively. After secondary ablation, however, pure fcc Ni nanoparticles were generated in methanol and hexane, while pure hcp Ni was formed in acetonitrile. We think that the solvent dependence on the phase of Ni nanocrystals is related to the specific heat of solvents which plays an important role kinetically and thermodynamically in the process of cooling the plasma plume where the nanoparticles nucleate and coalesce to a specific phase. The Ni nanoparticles prepared from PLAL were analyzed by X-ray diffraction (XRD) measurement, X-ray photoelectron spectroscopy (XPS), field emission-scanning electron microscopy (FE-SEM), high resolutiontransmission electron microscopy (HR-TEM), selected area electron diffraction (SAED), and fast Fourier transform (FFT) analysis.

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