RESUMO
The formation of Ag nanoparticles by electrochemical techniques has been investigated through a time-resolved UV-vis spectroscopy study. The formation of Ag(4)(2+) clusters is suggested as the main precursors to the particle formation. The mechanism also considers the electrodeposition which occurs as a parallel process in the electrochemical particle formation. Experiments at different current densities show that the electrodeposition is more important at low current densities. From the fittings of the change of the plasmon (lambda approximately 430 nm) and the cluster (lambda = 250 nm) bands to the proposed mechanism, the kinetic constants of the formation and disappearance of the Ag(4)(2+) cluster are derived. The kinetic fittings also allowed an estimation of the Ag(4)(2+) cluster extinction coefficient (epsilon(250) = 1.0 x 10(4) M(-1) cm(-1)). It is observed that the plasmon bandwidth (fwhm) follows the theoretical predicted 1/R law only for particles with sizes d approximately >3 nm, but the law is broken for the smallest particles (d < 2.5 nm). The break is associated with the existence of single-electron (SE) transitions which are activated by the plasmon decay for the smallest nanoparticles. From the broken 1/R law, a limit relaxation time of about 4 fs is derived for the plasmon deactivation. Below this limit, the plasmon seems to decay mainly through a nonradiative channel with the formation of electron-hole (e-h) pairs. By comparison of the 1/R broken law with other literature results, it is concluded that large interactions of the Ag nanoparticles with the used capping molecule (tetrabutylammonium acetate) facilitate the e-h plasmon deactivation.
