Synthesis of homogeneous alloy metal nanoparticles in silica aerogels

Homogeneous alloy nanoparticles of the noble metals gold and silver were formed in silica aerogels. In our synthesis procedure, silica aquogels were first prepared with a standard base-catalyzed route, washed several times with distilled water, and then bathed in an aqueous solution with a total metal ion concentration of 3 × 10−4 M. Formaldehyde (0.5 M) was then added as a reducing agent, with NaOH, in a typical concentration of 1–2 mM, employed to control the velocity of reduction. After reduction, the gels were washed, and then dried supercritically (from CO2) to yield aerogels. The optical absorption of the gels exhibits a single plasmon peak that shifts linearly to higher wavelengths when the Au mole ratio is increased. These results indicate that bimetallic nanoparticles are formed, and that the metals are homogeneously alloyed. Transmission electron microscopy shows that the diameter of the metal nanoparticles depends on the molar ratio of metals used in the bathing solution of the aquogel precursor, and ranges from a minimum of ∼20 nm (100% Au sample) to a maximum of 70–80 nm (25 at.% Au). Energy-dispersive X-ray analysis confirms that the nanoparticles are bimetallic, with a mean mole ratio within 15% of the bathing solution.