TITLE:

FROM ATOMS TO AGGREGATES: PROBING THE GROWTH OF SILVER NANOPARTICLES BY EXPERIMENT AND COMPUTATION

DATE:

Friday, April 15th, 2011

TIME:

3:30 PM

LOCATION:

GMCS 214

SPEAKER:

David Pullman, San Diego State University

ABSTRACT:

The increasing attention given to silver nanoparticles over the past decade stems primarily from their extraordinary optical and antimicrobial properties as well their ease of synthesis. Our lab is currently investigating 1) the use of silver nanoparticles to boost the efficiency of solar cells, 2) the effect of different “capping” ligands on the antimicrobial activity of silver nanoparticles, and 3) the chemical mechanism by which the nanoparticles are produced. The present talk will focus on very recent experimental and computational work regarding the last topic. Two methods of producing the nanoparticles in aqueous solutions will be discussed. The first method, by far the most common, uses sodium borohydride to reduce silver ions in the presence of a stabilizing ligand, such as citrate. Surprisingly, despite the abundance of studies of this reaction, its mechanism (as well as the net chemical reaction) are largely unknown. Through mass spectrometry and deuterium isotope studies, we show unambiguously that molecular hydrogen is a product and that the hydrogen is composed of atoms that originate in the borohydride and in water. Electronic structure computations (coupled-cluster and density functional) of small silver hydride molecules reveal that these are unexpectedly stable and may be intermediates in the reaction. The second method, developed in our lab, uses visible light to activate the growth of the nanoparticles from solutions consisting solely of silver nitrate and sodium citrate. A striking feature of this method is that the initial solution absorbs virtually no light at the incident wavelength. Since there must be at least one species that absorbs the light, we have directed our recent efforts on identifying possible candidate molecules. We again utilize electronic structure computations (equations-of-motion coupled-cluster), this time to simulate the UV-visible absorption spectra of possible silver complexes in solution. The results indicate that silver citrate may be responsible for absorbing light through a ligand-to-metal charge-transfer transition, leaving the silver ion nearly neutral and thus initiating the growth of the nanoparticles.

HOST:

Andrew Cooksy

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