A Simple Method for Preparation of Silica Aerogels Doped with Monodispersed Au Nanoparticles in Homogeneous Concentration

The capability to immobilize nanoparticles in a matrix preserving their unique properties has motivated research for several decades. Silica aerogels, being dielectric, transparent and with extremely low bulk density, were identified as promising candidates for this purpose. And yet, it was found difficult to obtain aerogels in which the particles were mono-dispersed and in homogeneous concentration.

Throughout the years, several techniques for preparation of such gels have been developed. However, in order to prevent aggregation of the particles, all these techniques rely on delicate manipulation of the gelation rate, or on precise timing of the particles` incorporation; thus making the techniques challenging to reproduce.

We propose a simple, yet robust method for preparation of doped aerogels, which requires only a minor modification of the one step aerogel recipe (involving: TEOS, ethanol, H₂O and ammonium catalyst). The dopants are implemented as silica coated nanoparticles suspended in ethanol. This suspension replaces the pure ethanol in the ethanol, H₂O and catalyst mixture; otherwise, the procedure follows the original aerogel recipe.

The prepared aerogels, with Au-nanoparticles with concentrations ranging from 50ppm to 4000ppm, exhibit homogeneous dispersion of the particles with no evidence of aggregation (as demonstrated by HRSEM images and absorption spectra). The presence of the particles does not affect the gelation process or the resulting silica matrix, with gelation time (~12min), bulk density (~80mg/cm³), pore size (4 – 40nm) and surface area (~900m²/g) being the same for all concentrations.

These results indicate that many other silica shelled nanoparticles can be implemented as dopants using the same methodology and in addition complex systems of several co-dopants can also be obtained in a reproducible manner with relative ease. Such aerogels can be used as tailor made elements in a wide variety of applications, in optics, electronics and medicine.