A nanoscale-modified LaMer model for particle synthesis from inorganic tin–platinum complexes† Samuel St. John,‡ Zhipeng Nan,‡ Naiping Hu,‡ Dale W. Schaefer‡ and Anastasios P. Angelopoulos‡ * The size-tunable structure and properties of Pt nanoparticles at the atomic length scale have attracted significant attention across a wide variety of fields including magnetics, electrocatalysis, optics, and gas- phase synthesis. Mechanisms responsible for the formation Pt nanoparticles remain unclear because of the difficulty generating in situ data for the time-evolution of size, shape, distribution, volume fraction, particle number density, and oxidation state from the starting complexes. We here demonstrate the use of simultaneous small- and wide-angle X-ray scattering combined with UV-vis spectroscopy to measure these key synthesis metrics for the reduction of Pt(IV) by Sn(II) in aqueous solution. This synthesis approach has been previously shown to permit continuous control over Pt nanoparticle size from 0.9 to 2.6 nm to within 10% standard deviation. Such fine control led to the discovery of densely packed amorphous structures at ca. 1.7 nm with substantially enhanced electrocatalytic oxygen reduction relative to nanocrystals and commercial electrocatalysts. Ex situ UV-vis and in situ X-ray scattering are here shown to reveal four distinct stages during synthesis: (1) autoreduction of a ligand/noble metal complex with a unique structure that depends on the Sn(II)/Pt(II) ratio, (2) generation of Pt primary particles and the formation of Pt nuclei at a rate that depends on the structure of the initial complex, (3) nanoparticle growth via LaMer's diffusion of these primary particles to the nuclei, and (4) growth termination due to capping from a stabilizing, two-layer ligand shell. We derive a set of consecutive rate equations and associated kinetic parameters that describe each step. The kinetics of ligand rearrangement has been previously found to limit the rate of nanoparticle growth. We incorporate this phenomenon into LaMer's classic diffusion-limited growth scheme to extend it to the nanoscale regime. This new model provides detailed understanding of how metal ligands serve as both reducing and stabilizing agents and allow for unprecedented, continuous control over both size and distribution. Systematic variation of temperature permits detailed time resolution at the very onset of Pt primary particle formation, as well as a means to determine temperature sensitivity of nanoparticle growth. 1 Introduction Direct control of individual atoms as a synthetic technique has fascinated scientists since the revolutionary 1959 lecture by Richard Feynman, “There's Plenty of Room at the Bottom”. 1 The synthesis of Pt nanoparticles is of particular interest in this context because of signicant opportunities for industrial applications in magnetics, 2 electrocatalysis, 3–5 optics, 6,7 and selective gas-phase synthesis. 8 Nanoparticle synthesis comes with many challenges. 9 Following Feynman, what is needed is precise tuning of nanoparticle properties through control of hierarchical assembly from atomic to microscopic scales. Current research, however, focuses on the equilibrated size and shape of Pt nanoparticles, missing the growth processes that determine size and shape. 10–13 Such kinetic data are essential to tune size-dependent characteristics. The paucity of kinetic data is due to two experimental requirements: (1) a reproducible synthesis scheme that produces monodisperse particles with sufficiently slow nucleation and growth to separate the stages of particle evolution; and (2) an in situ monitoring method that does not alter the growth mechanisms. Typical reducing agents such as borohydrides and hydrazine rapidly yield equilibrated structures aer only a few seconds. In addition, in situ char- acterization by electron scattering uses a low pressure envi- ronment atypical of colloidal synthesis, 14 while ultra-violet, visible-light (UV-vis) spectroscopy to monitor size and shape can only be applied to a few metals (e.g. silver) that exhibit Department of Chemical Engineering, School of Energy, Environmental, and Biological & Medical Engineering, University of Cincinnati, 693 Rhodes Hall, Cincinnati, OH 45221, USA. E-mail: anastasios.angelopoulos@uc.edu † Electronic supplementary information (ESI) available: Additional experimental methods and results including EXAFS ts, UV-vis extinction coefficients, as well as UV-vis and SAXS kinetic data at several additional temperatures are given in the ESI. See DOI: 10.1039/c3ta11552f ‡ The manuscript was written through contributions of all authors. Cite this: J. Mater. Chem. A, 2013, 1, 8903 Received 18th April 2013 Accepted 14th June 2013 DOI: 10.1039/c3ta11552f www.rsc.org/MaterialsA This journal is ª The Royal Society of Chemistry 2013 J. Mater. Chem. A, 2013, 1, 8903–8916 | 8903 Journal of Materials Chemistry A PAPER