Kinetics of Halide-Induced Decomposition and Aggregation of Silver Nanoparticles

We present a kinetic study of the effects of NaF, NaCl, NaBr, and NaI on aqueous solutions of 5 nm silver nanoparticles. There are distinct differences between these halides, which we attribute to two competing, halide-induced processes: oxidative decomposition of the nanoparticle surfaces and aggregation of the nanoparticles. NaF essentially does not react with the nanoparticle surface, but at concentrations above about 75 mM induces aggregation, albeit erratically. NaCl reacts oxidatively at concentrations below 27 mM, but the reaction is very slow because of surface passivation. The distinct, and lower, onset of aggregation at 27 mM is explained by chloride ion forming a uniform layer, which lowers the charge on the nanoparticle surface to about 2/3 of its original value. Addition of NaI or NaBr is very different; the rate of oxidative decomposition is orders of magnitude faster than that of NaCl such that the onset of aggregation is less apparent.