| Abstract
| - The reaction of sodium sulfide with chloroauric acid has been surrounded by a controversy over the structureof the resulting product. The original report proposed a Au2S/Au core/shell structure based on strong near-IRresonance and limited transmission electron microscopy. Subsequent reports used the same model withoutfurther attempts to determine the structure of the products. With a significant body of experimental workcompiled over a period of several years, we have shown that the major product of this reaction is aggregatedspherical nanoparticles of gold with a minority component consisting of triangular and rod-like structures.This is in contradiction to the core/shell structures as originally proposed. Recently, there have been additionalreports that again suggest a Au2S/Au core/shell structure or irregularly shaped Au nanoparticles as anexplanation for the near-IR resonance. To help resolve this issue, we have carried out further experiments todetermine how the reaction products may depend on experimental conditions such as concentration and agingof the reactants, particularly Na2S. It has been determined that sodium thiosulfate is the likely product fromNa2S aging. In addition, persistent spectral hole burning experiments have been conducted on gold nanoparticle aggregate (GNA) samples at excitation intensities that are lower than that required to melt thenanostructures. We have observed a decrease in optical absorption on resonance with the excitation laserwavelength, with simultaneous increases in absorption to the blue and red of this wavelength region. However,in the presence of the stabilizer poly(vinyl pyrrolidone) (PVP), no increase in absorbance was observed butrather a blue shifting and decrease in intensity of the near-IR plasmon resonance. These results imply that thenon-stabilized GNAs are able to break apart and reform into off resonant aggregate structures. In contrast,this behavior is suppressed in PVP stabilized GNAs because of the presence of polymer which quicklypassivates the individual nanoparticles that comprise the GNAs after they are disrupted by laser irradiation.These results would be very difficult to explain if the nanostructures were core/shell. Therefore, these newresults again support the model of GNAs as the best possible explanation for the product of the HAuCl4 andNa2S reaction.
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