Formation mechanism of noble metal nanoparticles in reactively sputtered TiO2 films

Abstract

Recently a simple recipe has been developed to prepare Ag nanoparticles in a TiO2 matrix {Okumu etal, [J. Appl. Phys. 97, 094305 (2005)] and Dahmen etal, [Appl. Phys. Lett. 88, 011923 (2006)]}. In this scheme, silver nanoparticles are formed in a TiO2 matrix by sputtering a thin silver film sandwiched between TiO2 layers, followed by an annealing process. To determine the formation mechanism of noble metal nanoparticles in the TiO2 matrix, we compare the behavior of Ag with two similar noble metals, gold and copper. The formation of metal nanoparticles in the TiO2 matrix is investigated using a combination of techniques including x-ray diffraction, x-ray reflectance, and optical spectroscopy. Despite the similarity of the three noble metals, no nanoparticles are formed from Cu and Au. This is in striking contrast to the behavior observed for Ag. The difference can be explained by a three step process, which involves oxidation of the metal, dissociation of the metal oxide upon annealing, and metal aggregation to form nanoparticles.