Optical study of redox process of Ag nanoparticles at high temperature

An optical study of the redox of Ag nanoparticles was carried out by loading the particles into the pores of monolithic mesoporous silica. The optical absorption of silver nanoparticles was measured after annealing in different atmospheres. It has been found that treatment in H2 or N2 induces a strong surface plasmon resonance (SPR) as usually expected, but air-annealing results in disappearance of such SPR. Alternate annealing in air and H2 (or N2) induces alternant disappearance and appearance of the SPR absorption, showing reversibility of optical changes. By additionally considering the surface and curvature effects of a particle, thermodynamic analysis for the oxidation of a Ag nanoparticle was conducted. Unlike bulk metal silver, which is stable and not oxidized in air at above 200 °C, for spherical Ag particle there exists a critical size for the oxidation (say, 2 nm at 500 °C). When the particle size is smaller than the critical value, oxidation could occur in air at above 200 °C and even at 700 °C. The reversible optical changes are attributed to the redox process of Ag particles in different atmospheres, which has been confirmed by further experiments.