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.

An experimental method is presented for fabrication of almost monodisperse metal nanoparticles on substrate surfaces. It relies on substantial narrowing of broad size distributions through irradiation with short laser pulses by exploiting the size dependent optical absorption coefficient of the metal particles. Successive irradiation by applying two laser wavelengths completely removes the smallest clusters of the distribution and causes a size reduction of the largest particles. Finally, only clusters with diameters in a very narrow size interval remain on the surface. By using this method, Ag clusters with mean diameters of 〈d〉=10 nm and size distributions with standard deviations of Δd/〈d〉=0.13 have been prepared.

Optical absorption of silver nanoparticles dispersed within pores of monolithic mesoporous silica was investigated 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 the disappearance of the SPR, which is contrary to the fact that bulk metal silver is stable and not oxidized in air at above 200 °C. The absence and appearance of the SPR absorption can be controlled through alternate annealing in air and H2 (or N2). Thermodynamic analysis for oxidation of Ag nanoparticles, by additionally considering the surface and curvature effects of a particle, shows the existence of critical particle size. Only when a Ag particle size is smaller than the critical value is the oxidation possible in air at above 200 °C. The optical changes are attributed to the occurrence of the redox of the Ag particles in different atmospheres at a high temperature.

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