Multicolour photochromism of TiO2 films loaded with silver nanoparticles

Photochromic materials, which change their colours reversibly in response to light, can be applied to smart windows, displays and memories1-3. Conventional photochromic materials respond in a monochromatic way, so that multicolour photochromism has required several different materials or filters combined appropriately. If multicolour photochromism could be achieved with a simple material, photochromic devices would be find a greater number of applications, including a rewritable colour copy paper or electronic paper 4 and a high-density multi-wavelength optical memory. Here we report multicolour photochromism of TiO2 films loaded with silver nanoparticles by photocatalytic means. Its colour, initially brownish-grey, changes under monochromatic visible light to almost the same colour as that of the light; the apparently uniform Ag-TiO2 film can be almost any colour. Behaviour similar to persistent hole-burning effects5,6 is also observed. The colour reverts to brownish-grey under ultraviolet light, and these processes are repeatable.