Optical Absorption of Copper Nanoparticles Dispersed within Pores of Monolithic Mesoporous Silica

Nano-sized copper particles are loaded into the pores of monolithic mesoporous silica by soaking and drying, followed by thermal reduction of copper nitrate [Cu(NO₃)₂]in a hydrogen atmosphere at 973 K for 90 min. It has been shown that copper nanoparticles are isolated from each other and highly uniformly dispersed inside the pores of silica. It is found that the surface plasmon resonance peak of the copper particles shows a significant red-shift with decreasing the particle size, which is in contrast to the corresponding fully embedded system. The peak decreases with exposure time to air and exhibits a linear relation with logarithmic exposure time. This red-shift phenomenon and decrease are explained on the basis of the structural features of this new composite or coexistence of local porosity and the nanoparticle's free surface in contact with air.