Anomalous phase transition and ionic conductivity of AgI nanowire grown using porous alumina template

Silver-iodide (AgI)-based superionic conductors are attracting widespread interest for their potential applications in electrochemical devices such as sensors and batteries. A new kind of nanocomposite with highly ordered AgI nanowires embedded in an anodic-aluminum-oxide (AAO) membrane was fabricated by low-temperature step-electrochemical growth. Structural evolution, phase transition, and ionic conductivity were investigated by x-ray diffraction, differential scanning calorimetry, and impedance measurements. The phase transition from beta/gamma-AgI phase to alpha-AgI phase occurred at temperature of 168 degrees C, that is, higher than that of reported bulk AgI (147 degrees C); abnormally, the alpha to beta/gamma phase-transition temperature on cooling was also depressed as large hysteresis formed. The high-temperature phase, namely, alpha-AgI, remained at temperatures as low as 80 degrees C. The initial highly oriented-growth AgI nanowire disappeared after undergoing heating and cooling processes and a mixture of polycrystalline beta/gamma-AgI and amorphouslike interface phases formed. The cooled AgI-AAO composite displayed ionic conductivity in the order of 10(-2) S cm(-1) at room temperature. This array-structured nanocomposite of AgI-AAO may be further developed for usage as a new type of battery, i.e., "nanobatteries" and "nanosensors" with individual AgI nanowires as basic elements.