León-Velázquez, Madeline S.

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    Nucleation and growth of silver sulfide nanoparticles
    (2010) León-Velázquez, Madeline S.; Castro-Rosario, Miguel E.; College of Arts and Sciences - Sciences; Mina, Nairmen; Cádiz, Mayra E.; Irizarry, Roberto; Suárez, O. Marcelo; Department of Chemistry; Santos, Jeannette
    The synthesis of metal and semiconductor nanoparticles has received widespread attention by the scientific community due to the large number of technological applications they find in nanoscaled systems. The mechanism of nanoparticle nucleation and growth is, in general, still an open question. We have employed transmission electron microscopy (TEM), scanning tunneling microscopy (STM), configuration interaction single calculations (CIS) and real time conductivity and UVvisible absorption measurements of stopped flows of AgNO3 and (NH4)2S to study the nucleation and growth for the formation of silver sulfide nanoparticles (Ag2S NP). Ionic conductivity measurements provide qualitative information related to the process occurring in the early stages of the reaction but does not provide information related to the identity and chemistry of the clusters or embryos chemistry. These conductivity measurements suggest the formation of non stoichiometric ionic Ag2S species, rather of neutral Ag2S, in the early stages of the reaction mixture. Comparison of the calculated absorptions spectra on several silver sulfide clusters with the optical absorption spectra of reaction mixtures obtained in the early stages of the reaction lead us to conclude that the dependence of the optical absorption spectra on time in the first few milliseconds after the flow stops is consistent with formation of AgS- and Ag3S + cluster ions. Bands that can be attributed to the formation of larger silver sulfide clusters – including Ag4S2 and Ag6S3 - are observed at longer reaction times: large amounts of the surfactant sodium bis(2-ethylhexyl) sulfosuccinate (AOT) inhibite the nucleation process and stabilize the D4h Ag2S4 cluster. The UV-visible absorption measurements at longer times revealed that the nucleation and growth process are not well separated in time. The initial nucleation and growth rates are found to increase with initial [AgNO3]0/[(NH4)2S]0 ratios larger than 1. Silver ions play a central role in the nucleation and growth process. The experimental results suggest that silver-rich-sulfides are involved in the nucleation stage and growth process of Ag2S NP.