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Preparation of sub-10 nm AgI nanoparticles and a study on their phase transition temperature.

Solid-state ionics have attracted great interest in many fields of science and technology owing to their properties and applications. Some of these ionic conductors exhibit high ionic conductivity comparable to liquid electrolytes coupled with high stability against heat or mechanical damage. One of the most fruitful results of studying the solid-state ionics was the discovery of the superionic conductivity of silver iodide (AgI). AgI is a mixture of b and g phases at ambient temperature and it undergoes phase transition into the a phase at 146 8C. The a-AgI is known as a superionic conductor and shows high silver ionic conductivity of over 1 Scm . The conductivity of the a-AgI is comparable to that of liquid sulfuric acid, and is even higher than that of liquid AgI. In the a phase, the iodide ions solely form the crystal sublattice, and the silver ions diffuse over the numerous sites that exist inside the sublattice of iodide. However, the highly ion-conductive phase appears only at high temperatures so far, and this makes it unsuitable for practical applications. Recently, nanomaterials have attracted a great deal of interest in scientific research and industrial applications. Their unique properties, based on their large surface-to-volume ratio and quantum size effect, are quite different from those of bulk materials. In particular, a nanosized material has a characteristic phase behavior, for example, the melting point of gold decreases with a decrease in the mean diameter. 5] Nanosized AgI, such as a nanoplate or nanowire, also shows a decrease in the phase transition temperature, which contributes to stabilizing the superionic phase at low temperature. According to these results, it is expected that small AgI nanoparticles (NPs) could be used to achieve superionic conduction at ambient temperature. To date, methods for the preparation of AgI NPs with diameters above 10 nm have been reported< ; however, to the best of our knowledge only one example of sub-10 nm AgI NPs has been described, in which the phase behavior is not clear. Herein, we report on the successful synthesis of sub-10 nm AgI NPs, and the high stability of the a phase. The AgI NPs were synthesized by treating silver nitrate (AgNO3) with potassium iodide (KI) in an aqueous solution in the presence of poly-N-vinyl-2-pyrrolidone (PVP) as a protective polymer (see the Supporting Information). Figure 1 shows a transmission electron microscope (TEM) image of the obtained AgI NPs and the size distribution. Their mean diameter was estimated to be 6.3 4.2 nm. To date, the size of AgI NPs is reported to be controlled in the range of 11–40 nm by using sodium iodide. We succeeded in preparing AgI NPs with a diameter of approximately 6 nm by using KI as an iodide source. Our results suggest that the potassium cation inhibits the growth of AgI nanocrystals, as PVP does. Similar cases have been reported for