Size dependent redox behavior of monolayer protected silver nanoparticles (2-7 nm) in aqueous medium

Monolayer protected nanoclusters are of current interest due to their ease of synthesis, high stability and possibility to precisely control their aspect ratio by preparation procedures, so that they can be tuned for a wide range of applications. Since these nanostructured metallic particles show fascinating size dependent optical, electronic, catalytic and magnetic properties, it is important to modulate their size, shape and intercluster spacing during their synthesis. These size dependent phenomena suggest that the electrochemistry of nanometer scale metal particles should be different from that of their bulk analogues. In the present study, we report a systematic variation in the redox behaviour of dodecanethiol protected silver nanoparticles with size (2-7 nm). Cyclic voltammograms in 0.1 M aqueous KCl solution show irreversible nature and the redox behaviour is indeed affected by the size as in agreement with the theoretical calculations of the Kubo gap. More specifically, the separation between oxidation and reduction peaks (ΔE_p) increases with an increase in size reaching a maximum (3.5-6 nm) followed by a decline, whereas the E_1/2 seems to be almost constant throughout this size regime. As the kinetic parameters are directly related to the ΔE_p value, the electron transfer facility should decrease with an increase in size in a similar manner. All the silver nanoclusters were characterized by their surface plasmon peak position, which was found to decrease with increase in size with a concomitant broadening. The particle size calculated from TEM reveals a fairly monodispersed nature whereas selected area electron diffraction (SAED) results confirm the presence of fcc structure for all the Ag clusters.