Charge storage and release onto Au and Ag nanoparticles in aqueous medium as probed by optical spectroscopy

Gold and silver nanoparticles in aqueous solutions can store negative or positive charges when, respectively, NaBH4, KI are introduced into the same media. The charge storage can be followed by the spectral shifts in the corresponding surface plasmon resonance (SPR) bands of these nanoparticles. In a similar way, the kinetics of these two processes can be monitored by the same shifts. Accordingly, we show that although Au nanoparticles exhibit smaller spectral shifts upon both negative and positive charge storage, when compared with Ag nanoparticles, their kinetics are faster towards reduction (electron storage) and comparable towards oxidation. Hence, if not the spectral shifts, the kinetics of the electron storage process can be correlated to the larger electron affinity of Au nanoparticles, when compared with those of Ag. The similarity of the kinetics towards oxidation must also be related to the small difference between the ionization potentials of Au and Ag nanoparticles.