Redox catalysis on "naked" silver nanoparticles

Silver nanoparticles in aqueous suspensions were synthesized by reduction of silver oxide by H2 following recently published procedures.12 These particles contain no foreign stabilizers or any ions other than those from silver and water. The particles were characterized by their UV−vis absorption spectra, by transmission electron microscopy, by dynamic light-scattering−zeta potential determination, and by the pH dependence of the potential. The nanoparticles are stabilized by adsorption of hydroxide anions at the particles surface. Their zeta potential is ∼35 mV at pH 9; it is pH dependent and the point of zero charge is near neutral. The negative potential, the hydrophilic surface that is created by the adsorbed hydroxide ions, which resembles an oxide interface, and the low ionic strength of the solution due to the small salt concentrations in this synthetic approach all contribute to the unusual stability of the colloidal suspension. The amount of molecular hydrogen formed was determined following γ-irradiation of the silver suspensions in the presence of 2-propanol and acetone. In these suspensions, all of the radiolytically generated radicals are converted to reducing radicals capable of reducing water to H2. At low doses, however, the amount is very small but after a "conditioning" period the yield substantially increases. The conditioning stage is explored in detail, and it is shown that the main reaction responsible for the conditioning is reduction of residual silver ions in the solution. The steady-state concentration of residual Ag+ ions that remains in the solution is determined by the relative rates of silver oxide dissolution and silver ions reduction during the synthesis. Analysis of the chemical composition of the suspensions at the end of the synthesis also indicates that a few percent of Ag+ ions remain unreduced. Most of the unreduced ions are free in the bulk solution while a minority adsorbs at the particle surface. Once the conditioning stage is complete, essentially all reducing radicals generated by the irradiation can catalytically produce hydrogen. For the presently studied silver particles of 30 nm, competition between radical−radical recombination and hydrogen evolution is suppressed at >2 mM silver concentrations. Little is to be gained by increasing the catalyst concentration above this level.