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small angle X-ray scattering

small angle X-ray

Article Cite This: Chem. Mater. 2018, 30, 1127−1135 pubs.acs.org/cm Tuning Precursor Reactivity toward Nanometer-Size Control in Palladium Nanoparticles Studied by in Situ Small Angle X‐ray Scattering Liheng Wu, †,‡ Huada Lian, ‡ Joshua J. Willis, ‡,§ Emmett D. Goodman, ‡,§ Ian Salmon McKay, ‡ Jian Qin, ‡ Christopher J. Tassone,* ,† and Matteo Cargnello* ,‡,§ † Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States ‡ Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States § SUNCAT Center for Interface Science and Catalysis, Stanford University, Stanford, California 94305, United States *S Supporting Information ABSTRACT: Synthesis of monodisperse nanoparticles (NPs) with precisely controlled size is critical for understanding their size-dependent properties. Although significant synthetic developments have been achieved, it is still challenging to synthesize well-defined NPs in a predictive way due to a lack of in-depth mechanistic understanding of reaction kinetics. Here we use synchrotron-based small-angle X-ray scattering (SAXS) to monitor in situ the formation of palladium (Pd) NPs through thermal decomposition of Pd−TOP (TOP: trioctylphosphine) complex via the “heat-up” method. We systematically study the effects of different ligands, including oleylamine, TOP, and oleic acid, on the formation kinetics of Pd NPs. Through quantitative analysis of the real-time SAXS data, we are able to obtain a detailed picture of the size, size distribution, and concentration of Pd NPs during the syntheses, and these results show that different ligands strongly affect the precursor reactivity. We find that oleylamine does not change the reactivity of the Pd−TOP complex but promote the formation of nuclei due to strong ligand−NP binding. On the other hand, TOP and oleic acid substantially change the precursor reactivity over more than an order of magnitude, which controls the nucleation kinetics and determines the final particle size. A theoretical model is used to demonstrate that the nucleation and growth kinetics are dependent on both precursor reactivity and ligand−NP binding affinity, thus providing a framework to explain the synthesis process and the effect of the reaction conditions. Quantitative understanding of the impacts of different ligands enables the successful synthesis of a series of monodisperse Pd NPs in the broad size range from 3 to 11 nm with nanometer-size control, which serve as a model system to study their size-dependent catalytic properties. The in situ SAXS probing can be readily extended to other functional NPs to greatly advance their synthetic design. 1. INTRODUCTION Rational design and synthesis of well-defined colloidal nanoparticles (NPs) is of utmost importance for fundamentally studying their intrinsic properties and for various technological applications. 1−3 Over the past two decades, the synthetic control of colloidal NPs has come to the level that by tuning reaction conditions (e.g., reaction precursors, ligands, reaction temperature, etc.), various sizes, shapes, compositions, and even complex structures of NPs have been achieved. 4−8 However, these syntheses are typically developed empirically, using trialand-error approaches that cause substantial waste of time and resources. Understanding the formation mechanism of these NPs will provide guidelines for greatly accelerating their synthesis with tailored properties. Unfortunately, it is still challenging to study the fast nucleation and growth kinetics due to the lack of proper experimental setups. 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. 9,10 Alternatively, synchrotron based X-ray scattering techniques, due to high penetration of the X-ray and fast data aquisition, 11,12 have advanced our understanding of NP nucleation and growth during colloidal synthesis. 13−37 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. 16 The growth mechanism of Au NPs was also studied, 18,19 which involves a rapid nucleation followed by NP growth driven by both monomer attachment and particle coalescence. This in situ technique has also been utilized to experimentally probe NP formation under harsh reaction conditions, such as the formation of CdSe quantum dots at 240 °C in a glass capillary, in which the thermal activation of selenium precursor was found to be the growth rate-determining step. 24 Despite this Received: December 14, 2017 Revised: January 2, 2018 Published: January 3, 2018 © 2018 American Chemical Society 1127 DOI: 10.1021/acs.chemmater.7b05186 Chem. Mater. 2018, 30, 1127−1135

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