Abstract
Abstract
Stable silver clusters can be prepared by a simple electroless reduction reaction taking place in water-in-oil emulsions. An emulsion containing AgNO3 in the water droplets was mixed with a similar emulsion containing aqueous NaBH4 droplets. The droplet diameter, based on Rayleigh scattering, was 41 nm and the mean number of Ag+ ions in each droplet varied from 2.0 to 21.7 as the concentration increased from 90 μM to 1 mM AgNO3. The low number of Ag+ ions in each droplet inhibits the growth of large nanoparticles and these emulsions do not show the large plasmon band observed for Ag nanoparticles obtained by the analogous reaction in bulk solution at the same Ag+ concentrations. Atomic force microscopy provides evidence of small Ag nanoclusters and a much lower number of larger nanoparticles. Electrospray mass spectrometry suggests that the clusters are mainly Ag4 species coordinated to water and BH4
−. The Ag nanocluster-containing emulsions are fluorescent and show an emission band with a peak wavelength of 427 nm and a Stokes shift of 81 nm from the first peak at 346 nm in the excitation spectrum. The intensity of fluorescence decreased as the [Ag(I)] increased and our most fluorescent samples were prepared from 90 μM AgNO3 because at higher concentrations more Ag nanoparticles are formed. DFT calculations on Agn clusters indicated that Ag4 species favour a planar rhombic geometry even in the presence of coordinating water molecules or BH4
−. However calculations of vertical excitation energies for Ag4 species do not match the experimental excitation spectra and this suggests the fluorescence arises from bright AgNCs of different nuclearity present at lower abundance in the mixture of species produced by the emulsion reaction. Calculated excitation energies for Ag6 give the best fit to the available data.