Changes in the shape and optical properties of gold nanoparticles contained within alumina membranes due to low-temperature annealing

The shape and optical properties of Au nanoparticles electrochemically deposited within the pores of porous alumina membranes have been studied as a function of the temperature to which the Au/alumina composite was heated. Low aspect ratio Au nanoparticles begin as thin skinned particles, essentially flakes of Au within the pores, but after heat treatment at low temperatures (<400 degrees C), they anneal to become dense, roughly spherical particles. Visually, the color of these membranes changes from blue to red after the thermal treatment. In contrast, the nanoparticles with an aspect ratio of ca. 3 exhibit essentially no change in either optical absorption characteristics or particle shape after exposure to the heating program. These membranes were initially red and did not change color with heating.