A general approach to prepare nanodispersions of noble metals in organically modified silicates is presented with particular emphasis on the synthesis of gold nanodispersion and its characterization. Organically modified sol−gel monomers containing amine functional groups are used to stabilize the metal salts before the reduction step and to cap and prevent coagulation of the metal sol after reduction and through gel formation, drying, and aging. Uniform spherical metal nanodispersions of Au, Pt, and Pd with average diameter of 4−6 nm were obtained. The particle size distribution and the average size of silver nanoparticles prepared by the same method were considerably larger. This was attributed to the lower stability of the silver−amine complex and to the lower affinity of amines to silver surfaces. Stable aqueous colloidal solutions and supported metal nanodispersions in porous films and monoliths can be prepared by this route.

Electronic transport in gold−dithiol nanoparticle films was studied using conductivity, photoconductivity, and photoelectrochemical means. The films were characterized by SEM and optical spectroscopy. GC/MS was used for the analysis of the pyrolysis products during heat treatment. Films were assembled on glass substrates using gold sol and different alkanethiol spacers (1,2-ethanedithiol (C2), 1,5-pentanedithiol (C5), and 1,8-octanedithiol (C8)). Resistance−temperature measurements revealed that the effective activation energies for conduction were 0, 5, and 15 meV for films assembled using C2, C5, and C8 spacers, respectively. Light action spectra of photoconductivity of gold−dithiol nanoparticle films revealed 0.8−1.0 eV threshold photon energy. The difference between the observed threshold energies points to different mechanisms for conductivity and photoconductivity. The low effective activation energy for dark conduction is attributed to a mixed mechanism of conduction, tunneling between insulated particles, and metal conduction through defects which are ascribed to direct contact points between metal particles. The photoconductivity mechanism involves photoemission from metal particles into the insulator layer. Photoelectrochemical studies of gold nanoparticle electrodes in aqueous electrolyte revealed 3.5 eV photon energy threshold of the photocurrent at an electrode potential of E = 0 V vs Ag/AgCl reference. The much higher photoelectrochemical threshold energies are ascribed to direct photoemission processes from the surface metal particles into the electrolyte. Heat treatment of the films decreased film resistance and increased the temperature coefficient of resistance to values approaching that of metal gold. These trends are attributed to pyrolysis of spacer molecules, which favor the metal conduction mechanism.

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