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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The physical and chemical properties of TiO 2 have been of great interest due to the potential application in photocatalytic, photovoltaic, lithium-ion battery and dye-sensitized solar cell systems.1-3 Especially, its characteristic properties in optical and electronic properties, nontoxicity, chemical inertness, and high melting temperatures have drawn extensive research on the ability of semiconductor photocatalyst to promote the photodegradation of various pollutants. 4,5Anatase, brookite, and rutile are the major crystalline structures of TiO 2 , of which rutile phase is most stable; whereas anatase and brookite phases are metastable and easily transformed to rutile phase when heated above about 600-8006,7 As demonstrated in other experimental studies, the anatase form of TiO 2 has shown much higher photoactivity than that of rutile. 3,5,8,9 In the meantime, a number of methods have been explored and developed for the synthesis of TiO 2 nanoparticles, such as hydrolysis of Ti(IV) ions, 10 hydrolysis of titanium alkoxides 11 or titanium tetrachloride 12 in gas phase, sol-gel, 13hydrothermal hydrolysis, 14 and precipitation, 15 of which many studies were devoted to the synthesis of rutile phase, while fewer investigations on the anatase phase. This naively supports that synthesis of pure anatase phase is more challenging than that of rutile. Furthermore, the major products from the above techniques are typically amorphous and require post-calcination for better crystallinity and morphology of the product, which inevitably results in the formation of larger sizes of TiO 2 nanoparticles. Therefore, it is demanding to explore a method that overcomes the problems mentioned above.In order to fabricate small-sized TiO 2 nanoparticles with good crystallinity at room temperature, we have applied a pulsed laser to ablate a Ti plate immersed in deionized water and methanol. Pulsed laser ablation in liquid (PLAL) 16 was introduced by Patil and co-workers in 1987 and has been demonstrated to be an effective, simple, and versatile method for synthesizing nanomaterials starting from an appropriate choice of solids and liquids.17-20 Because of relatively simple and clean preparation of nanoparticles using PLAL, coupled with its versatility, many different metal nanostructures providing desired functions have been fabricated. One of the main advantages of PLAL compared to other methods is that it can generate nanomaterials without the need of any catalysts or organic additives. Hence, in recent years, PLAL has become a successful nanomaterial fabrication tool for the formation of the novel nanostructures.19,21-25 However, compared to large investigations on fabrication of TiO 2 by laser ablation in vacuum, 7,26-31 there were few studies on the preparation of TiO 2 nanoparticles by PLAL.32-34 Furthermore in the previous PLAL studies, the synthesis of rutile and mixture of anatase and rutile phase was mostly employed. 34,35In the present note, we report a one-step synthesis of pure anatase TiO 2 nanoparticles by PLA of Ti pla...