Shape-controlled synthesis of silver nanostructures for high-thermal conductivity nanofluids

A nanofluid is a solid-liquid composite material consisting of a stable suspension of nanometric particles in a conventional refrigerant liquid expected to exhibit enhanced heat transfer properties. Elemental silver (Ag) was selected in this research because of its high electrical and thermal conductivity that are likely to be dependent on the crystal size and shape at the nanoscale. Accordingly, we have synthesized highly monodisperse silver nanowires and nanocrystals by reducing silver nitrate solutions with ethylene glycol in presence of polyvinylpyrrolidone, (PVP). The shape-control in the silver nanostructures was achieved by a proper selection of the type and level of chloride salts, e.g. KCl and CaCl2, and specific PVP/Ag mole ratios in starting solutions. The development of the metal phase was confirmed by X-ray diffractometry. Transmission electron microscopy analyses evidenced the formation of silver nanowires exhibiting a very uniform thickness that could be tuned in the 40-130nm range. UV-vis measurements evidenced the plasmon peak at ∼387nm and clear shoulders at ∼357nm that are indicative of the formation of elongated nanostructures.