Rate-Redox-Controlled Size-Selective Synthesis of Silver Nanoparticles Using Polyoxometalates

Uniform silver nanoparticles were obtained upon the reduction of silver ions by reduced polyoxometalates (POMs) by simple mixing at room temperature. Control of the size and the dispersity of silver nanoparticles was achieved by rate control. Faster reduction of Ag+ led to smaller and more uniform silver nanoparticles, suggesting that the rate of Ag+ reduction strongly affects the initial nucleation of silver particles. A faster rate is achieved by either (i) increasing the concentration of the reducing reagent: (reduced POM) or, more interestingly (ii) selecting POMs having appropriate (more negative) redox potentials: different POMs with increasing negative reduction potential or the same POM with more electrons accumulated on it. The rates of Ag+ reduction parallel the more negative reduction potential for POMs according to the series H2W12O407- > SiW12O405- > P2W18O628- > P2W18O627- > P2Mo18O6210- > P2Mo18O628- = 0. This precise redox-control is summarized in a log k (E-0) electrochemical-like behavior, in alliance to a soluble cathode behavior of POMs, which leads to more uniform particles than those obtained with conventional electrochemistry. Alternatively, the concentration of silver ions influences the size of the particles obtained: increasing the amount of Ag+ leads to larger nanoparticles. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)