Hydroxylamine seeding of colloidal Au nanoparticles. 3. Controlled formation of conductive Au films

An approach to enlarge preformed colloidal Au nanoparticles in solution based on the Au colloidal surface-catalyzed reduction of Au³⁺ by NH₂OH ('seeding') has been adapted to 12-nm-diameter colloidal Au nanoparticles immobilized in monolayers and multilayers. Bulk characterization of the ensuing films was carried out by atomic absorption, UV-vis-near-IR optical spectroscopy, cyclic voltammetry, and dc resistance measurements. Exposure of a 12-nm-diameter Au colloid monolayer on organosilane-modified glass surfaces to NH₂OH/Au³⁺ leads to rapid particle growth and coalescence: after roughly 5-10 min, the optical and electrical properties closely resemble that of conductive Au thin films prepared by evaporation. Evolution of the nanometer-scale architecture was followed using atomic force microscopy (AFM), surface-enhanced Raman scattering (SERS), surface plasmon resonance (SPR), and field emission scanning electron microscopy (FE-SEM), leading to the following key findings: (i) Seeding of surface-confined colloidal Au leads to a very different distribution in particle size/shape than seeding with identical particles in solution. (ii) Au³⁺/NH₂OH-mediated growth of surface-confined colloidal Au is highly dependent on particle coverage, with different particle densities in monolayers leading to distinct film architectures that are easily detected by FE-SEM and SPR.