Elucidating the mechanism of silver particle aggregation is important in surface enhanced Raman spectroscopy because of the greater enhancement associated with aggregated particles.  Also, aggregation is an important process that has to be considered in environmental and biological systems.  It is well known that adding salts to a colloid solution affects the aggregation process, not only because of the increase in ionic strength, but also, at times, because of adsorbate exchange which can affect the nanoparticle surface directly.  This is particularly true of chloride ions, and we have focused on studying the effects of this ion by monitoring the size and shape of the surface plasmon resonance (SPR) peak near 400 nm.              We have measured the temperature dependence of the rate of chloride-induced nanoparticle aggregation between 0oC and 40oC, and have found a negative apparent Arrhenius activation energy of about -70 kJ/mol.  Although calculations of bimolecular rate constants using DLVO theory can show an inverse temperature dependence, the apparent activation energy is generally small and does not explain the large values that we observe.  This indicates that the mechanism is complex with the initial bimolecular association not being the rate-limiting step.               Above 40oC, surface decomposition appears to dominate the rate of decrease of the SPR peak, so we have measured the temperature dependence of the decomposition rate by adding NaCl at lower concentrations where no aggregation takes place.  This rate has been found to increase significantly with increasing temperature, and can compete with particle aggregation in reducing the amount of silver found in single nanoparticles.

Mechanism of chloride-induced silver nanoparticle aggregation

248th National Meeting (2014)

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