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Direct preparation of sulfide semiconductor nanoparticles from the corresponding bulk powders in an ionic liquid.

The physicochemical properties of materials depend on their particle size. Appealing properties can be imparted to different materials at nanometric levels, thus generating a new range of interesting and promising products with different optical, electronic, magnetic, chemical, and mechanical properties from those of the bulk materials. In particular, semiconductor nanoparticles, and more specifically chalcogenide nanoparticles, have been intensively studied because of their quantum confinement effects and size-dependent photoemission characteristics. These semiconductor nanoparticles are widely used for biological labeling and diagnosis, light-emitting diodes, electroluminescent and photovoltaic devices, lasers, single-electron transistors, and catalysis. Over the last decades, a great effort has been made in the development of approaches for the synthesis of nanoparticles with controlled size and shape, as well as on the study of their properties. Among these approaches, liquid-phase methods (which comprise both microemulsion and reaction techniques) are the most relevant because of their simplicity and ability to control the nanoparticles morphology according to the operation conditions. Ionic liquids are salts with low (< 100 8C) melting temperatures. They typically exhibit properties such as extremely low vapor pressures, wide liquid ranges, ability to dissolve a broad variety of compounds, and good thermal and chemical stabilities. Moreover, by judicious selection of the cation– anion combination (“design” of the ionic liquid), it is possible to adjust the properties of the ionic liquid to a considerable extent to match those required for a given application. These characteristics render them as interesting compounds for the development of more sustainable processes in a great variety of applications in different fields. Subsequent to a burgeoning in research on ionic liquids in the late 1990s, they were first used in a method for the preparation of nanoparticles about a decade ago. Since then, it has been proposed that ionic liquids may provide both steric and electrostatic stabilization to nanoparticles, and they have been used in a variety of roles (e.g., (co)solvents, reactants, templates) in a good number of methods for the synthesis of inorganic nanoparticles with novel morphologies and improved properties. Nanomaterials preparation methods involving ionic liquids have been mostly applied to the synthesis of metal nanoparticles, although the preparation of metal oxide nanoparticles is gaining increasing attention. Also, other nanoparticles of interest such as metal chalcogenides, in particular sulfides, have been synthesized using methods based on ionic liquids. For example, CdS and PbS nanoparticles were prepared in the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate, using ultrasonic irradiation, with thioacetamide and the corresponding metal acetate as precursors. In another example, ZnS nanoparticles were prepared from zinc acetate in the ionic liquid 1-butyl-3methylimidazolium tetrafluoroborate, by a microwaveassisted method. In spite of the introduction of ionic liquids in liquid-phase methods for the preparation of nanoparticles, the utilization of organic solvents and/or solid precursors, which have associated undesired effects from a perspective of sustainability, has remained necessary. Herein, we present a novel method (Figure 1), in which only an ionic liquid and the bulk powder of the material of the target nanoparticle are used,