Recent remarkable progress in nanomaterial research led to the development of new materials with unusual features. We report on the properties of a composite material made of silver nanoparticles embedded in nanoporous TiO₂ film in which unique multicolour photochromic behaviour was observed [1, 2]. The colour of the material can be changed by light irradiation. The original hue gray film surface appears to change its colour in accord with that of the illuminating light. We prepared our samples by UV photocatalytic reduction of Ag⁺ ions to Ag nanoparticles in nanoporous TiO₂ films. Different shapes and sizes of nanoparticles result in a wide inhomogeneous broadening of their extinction peak over the whole visible spectral region. Spectrally narrow light interacts selectively with some of the nanoparticles leading to a reduction of the film extinction in the spectral region of illuminating light (“hole-burning”), see Fig. 1. We report on the optimization of preparation of photochromic Ag- TiO₂ films and on their properties as studied by optical spectroscopy. We discuss also the microscopic model of the observed behaviour. The authors acknowledge the financial support of the Grant Agency of Charles University in Prague (grant No. 130607) and of the Ministry of Education of the Czech Republic under the research plan MSM 0021620834.

[1] Ohko Y., Tatsuma T., Fujii T., Naoi K., Niwa C., Kubota Y., Fujishima A., Nat. Mater. 2 29 (2003)

[2] Okumu J., Dahmen C., Sprafke A. N., Luysberg M., von Plessen G., Wuttig M., J. Appl. Phys. 97, 094305 (2005)

Fig. 1 – Changes in the extinction spectra (normalised) of nanocrystalline Ag-TiO₂ films after irradiation by six different laser lines indicated by arrows.

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