Optical Absorption of Copper Nanoparticles Dispersed within Pores of Monolithic Mesoporous Silica
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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.
I. INTRODUCTION
II. EXPERIMENTAL
Loading nanometer particles (semiconductors, metals, or compounds) into the pores of mesoporous solids will form new composite materials. These new composite materials display many unique properties and have received considerable attention in recent years.1–5 In our previous works, monolithic mesoporous silica loaded with silver and gold nanoparticles exhibited novel properties such as the optical switching effect, reversible optical phenomenon, and the ambient-induced interface coupling effect.6–8 It has been found that glasses containing metal crystallites (such as Ag, Au, and Cu) showed an enhanced third-order susceptibility and have applications in nonlinear optics and optical switching devices.9,10 Recently, we prepared copper nanoparticles dispersed into pores of monolithic mesoporous silica. From the optical absorption spectra, we found that the surface plasma resonance peak, which is directly related to third-order susceptibility, of copper colloid particles displayed a considerable red-shift with decreasing particles size. This is in contrast to that observed in the case of copper particles fully embedded in glass or solid argon matrices, in which either a slim blue shift or no obvious shift was seen.11,12 The details are reported as follows.
Monolithic porous silica (planarlike in shape and about 1 mm in thickness) was prepared from the tetraethyl orthosilicate, alcohol, and distilled water by the sol-gel technique, drying and finally annealing in air at 973 K for 60 min, as previously described.3 The porosity of the monolithic silica obtained in this way was estimated to be about 50%. The preformed sample was then soaked in Cu(NO3)2 solution (0.06 mol/l) at room temperature. After a sufficient soaking time (about 2 weeks), the sample was taken out and dried at 393 K to remove the solvent from the pores of silica and finally annealed in H2 gas at 973 K for 90 min to reduce the copper nitrate into copper particles within the pores of silica forming the Cu/SiO2 composite. Additional annealing for the sample was conducted at 773, 873, 973, and 1073 K respectively, in 5% H2–95% N2 mixed atmosphere for 150 min, followed by cool
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