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Kinetics of the decomposition and aggregation of silver nanoparticles induced by halide ions
Espinoza, Maria Gabriela
Pullman, David PTong, WiliamAnderson, Matthew
Oxidative decomposition is observed at lower halide concentrations where the rate order of the reaction is 2/3. The more thermodynamically favorable silver halide has the highest rate of decay: I⁻ > Br⁻ > Cl⁻ > F⁻. The rate of nanoparticle aggregation increases substantially when the ionic strength of the solution increases. This is due to an increase in the screening of the electrostatic force between the particles. There is considerable interest in nanoparticle aggregation because it intensifies the Surface-Enhanced Raman Spectrum of species absorbed on the nanoparticle surfaces. Chloride ion is typically used for this purpose, but, according to the Derjaguin, Landau, Verwey and Overbeek theory, the aggregation rate of a particular type and size of nanoparticle depends only on the ionic strength, so any salt should have the same effect. Yet, we find that chloride ion and fluoride ion have very different critical coagulation concentrations -- the concentration at which the aggregation rate increases to the point of being diffusion-limited. We have measured aggregation rate constants as a function of halide concentration and compared them with those calculated using a pair-potential which combines a van der Waals attractive potential with an electrostatic repulsive potential. This comparison shows that the different effects of the two halide ions are likely due to their effect on the nanoparticle surface charge. The decay reactions were also studied under various temperatures to observe any changes in the rate decay at the different temperatures. Details of the experiments and analysis which lead us to this conclusion will be presented.
Master of Science (M.S.) San Diego State University, 2012
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