Optical absorption of Ag oligomers dispersed within pores of mesoporous silica

Small Ag particles were obtained by thermal decomposition of silver nitrate within pores of mesoporous silica in H2 at temperatures from 100 to 700 °C. For the sample treated at 100 °C, the absorption spectrum exhibits three absorption peaks including both surface plasmon resonance peaks at 400 nm from Ag nanoparticles and two new peaks located at 300 and 330 nm which are attributed to the Ag oligomers. Increasing the temperature, the absorption from oligomers become weakened and finally vanishes, whereas the absorption peak at 400 nm becomes gradually dominating. When exposing the fresh sample to air, an obvious depression following an enhancement in absorption is found, which is considered to result mainly from oxidation of Ag particles. The lifetime of these oligomers can reach as long as 4 days in air, which may be due to the formation of H-bonds and the slow diffusion of oxygen in the confined pores.