Time-Resolved, in Situ, Small- and Wide-Angle X-ray Scattering To Monitor Pt Nanoparticle Structure Evolution Stabilized by Adsorbed SnCl₃^– Ligands During Synthesis

Abstract

Understanding and controlling nanoparticle formation mechanisms is important because of increasing reports of unusual material properties in this critical size region in fields ranging from magnetics, electrocatalysis, optics, and heterogeneous synthesis. Here we use real-time, in situ smallangle and wide-angle X-ray scattering to dynamically monitor the production of Pt critical nuclei from Sn−Pt complexes on a length scale approaching 0.6 nm. A time resolution of 20 s is achieved due to the slow reduction and growth processes of this unique system. The structure and evolution of inorganic Sn ligands on these Pt nanoparticle cores is of particular interest because of the ability of Sn to mitigate poisoning of electrocatalyst surfaces by adsorbed CO intermediates (CO ads ) on the anode of direct alcohol fuel cells. Coherent scattering from the stabilizing ligand shell is identified in the wide-angle scattering data and suggests segregation of Sn ligands at the surface of the nanoparticles synthesized using our approach. We correlate the autoreduction kinetics of precursor complexes with number density, polydispersity, and Pt volume fraction, thereby establishing the mechanism of particle size control at the onset of atomic coalescence. Particle growth is shown to terminate when a Sn shell of well-defined thickness (about 0.6 nm) is completed for a range of Pt core diameters in this critical size range.

Insights into Reaction Intermediates to Predict Synthetic Pathways for Shape-Controlled Metal Nanocrystals

“…Monodisperse metal nanocrystals (NCs) with well-defined size and shape continue to offer new opportunities in current materials research, spanning from catalysis to energy conversion and biomedicine. − Colloidal chemistry is a powerful approach to access NCs with a level of tunability which is superior to other techniques. − Mostly developed by a trial-and-error approach, this synthetic route commonly employs metal salts as precursors, often a reducing agent, organic ligands, or surfactants, to stabilize the final NC product and a carrier solvent. − The size and shape of the obtained NCs are sensitive to several parameters, including temperature, atmosphere, reaction time, overall volume of the reaction mixture, as well as the time and rate of injection of additional reactants. − Despite the numerous reported procedures, the understanding of the reaction pathways and how they relate to the NC size and shape remains limited. General guidelines and theoretical descriptions of reaction parameters/size/shape relations are reported in the literature. − In recent years, the development of in situ techniques based on X-ray spectroscopy and electron microscopy has enabled progress in the understanding of NC formation. − However, most of these studies focus on low temperature syntheses in aqueous solution, which are easier to monitor without sophisticated in situ setups. Furthermore, most of them describe the postnucleation stage and aim at corroborating theoretical models for size control rather than for shape control. …”

“…General guidelines and theoretical descriptions of reaction parameters/size/shape relations are reported in the literature. − In recent years, the development of in situ techniques based on X-ray spectroscopy and electron microscopy has enabled progress in the understanding of NC formation. − However, most of these studies focus on low temperature syntheses in aqueous solution, which are easier to monitor without sophisticated in situ setups. Furthermore, most of them describe the postnucleation stage and aim at corroborating theoretical models for size control rather than for shape control. − …”

Tuning Precursor Reactivity toward Nanometer-Size Control in Palladium Nanoparticles Studied by in Situ Small Angle X-ray Scattering

“…In past few years, the fast developments of in situ experimental techniques have enabled the real time probing of NP formation in solution. Direct visualization of NP growth at the atomic resolution has been realized by in situ transmission electron microscopy (TEM) using liquid environmental cells. , Alternatively, synchrotron based X-ray scattering techniques, due to high penetration of the X-ray and fast data aquisition, , have advanced our understanding of NP nucleation and growth during colloidal synthesis. − Taking Au NPs as an example, using in situ small-angle X-ray scattering (SAXS), the nucleation kinetics of Au NPs synthesized using different ligands were directly monitored . The growth mechanism of Au NPs was also studied, , which involves a rapid nucleation followed by NP growth driven by both monomer attachment and particle coalescence.…”

A small-angle scattering environment forin situultrasound studies

“…Second, use the ultrasound sample environment in time-resolved scattering instruments. 64–68 Free electron X-ray lasers and synchrotron sources now have the ability to resolve at up to femtosecond time resolutions, while SANS techniques that resolve millisecond time scales are also possible. 60–63 Depending on the acoustic conditions used, bubble lifetimes can vary from microsecond to millisecond time scale.…”