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Silver Nanoparticles Synthesized by Microwave Heating: A Kinetic and Mechanistic Re-Analysis and Re-Interpretation

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journal contribution
posted on 01.12.2017, 14:49 by Saim Özkar, Richard G. Finke
A quantitative kinetics and mechanistic re-analysis is performed of an important 2016 paper that described the formation of Agn nanoparticles from the polyol reduction of silver nitrate in the presence of poly­(N-vinylpyrrolidone) under microwave heating. Elegantly and expertly obtained, in operando synchrotron high-energy X-ray diffraction (HEXRD) data, integrated with the microwave heating for the first time, were used to follow the Agn nanoparticle formation reaction in real time and to obtain time-resolved, HEXRD peak areas for the formation of both Ag(111) and Ag(200) facets. Unfortunately, the subsequent kinetics and mechanistic analysis that resulted is far from the state-of-the-art and was done without citing nor using well-established literature of nanoparticle nucleation and growth kinetics and mechanisms that has been available for over 20 years. Herein, the data are re-analyzed and re-interpreted in light of the fitting of the kinetics data with the presently most widely cited and employed, deliberately minimalistic, disproof-based nanoparticle nucleation and growth mechanism, dating back to 1997, of the pseudoelementary steps of slow continuous nucleation, A → B (rate constant k1), and then fast, autocatalytic surface growth, A + B → 2B (rate constant k2), where A is the starting Ag+ and B is the Ag0 product. The two pseudoelementary step mechanism is shown to be able to account for the previously reported kinetics data even for these large, up to ∼100 nm (i.e., 0.1 μm) Agn nanoparticles, a remarkable result in its own right given that there are on the order of ∼107 Ag(0) atoms in a ∼100 nm particle formed from the reduction of ∼107 Ag+ atoms in what must be >107 actual elementary steps. However, the k2 rate constant in particular likely loses much of its value since it is an average over a large change in the percentage of surface atoms in the growing nanoparticle. The results lead to nine revisions of questionable to incorrect previous claims and conclusions, plus a series of eight insights and guidelines for future work in nanoparticle formation kinetics and mechanism. An extensive Supporting Information further discusses interesting questions regarding the issues in analyzing and understanding nanoparticle formation kinetics and the mechanism of such large, ∼0.1 μm-sized particles.