Article pubs.acs.org/JPCC
Real-Time Probing of the Synthesis of Colloidal Silver Nanocubes with Time-Resolved High-Energy Synchrotron X-ray Diffraction Sheng Peng,† John S. Okasinski,‡ Jonathan D. Almer,‡ Yang Ren,‡ Lin Wang,§ Wenge Yang,§ and Yugang Sun*,† †
Center for Nanoscale Materials, and ‡X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States § HPSynC, Geophysical Laboratory, Carnegie Institution of Washington, 9700 South Cass Avenue, Argonne, Illinois 60439, United States S Supporting Information *
ABSTRACT: Understanding of the nucleation and growth mechanism of colloidal nanoparticles is of key importance for better design and synthesis of nanomaterials with precisely tailored properties. Such mechanistic studies require in situ techniques that probe the complex chemical and physical events involved in the formation of nanoparticles in real time. Here, we report the use of high-energy synchrotron X-ray beam as a unique probe to monitor the nanophase evolution involved in the synthesis of colloidal Ag nanocubes. Timeresolved X-ray diffraction (XRD) reveals that at least three nucleation and growth processes occur sequentially: the formation of AgCl nanocrystals, the formation of multipletwinned Ag nanocrystals, and the solid-phase transition of the AgCl nanocrystals to single-crystalline Ag ones. In addition, quantitative analysis of the XRD patterns advances the understanding of reaction kinetics involved in these nucleation and growth processes. This time-resolved in situ technique can be applied to a great variety of solution-phase reactions for the synthesis of colloidal nanoparticles.
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INTRODUCTION Controlled synthesis of colloidal nanoparticles has been extensively studied to achieve enhanced performance of the nanoparticles in applications by tailoring their unique properties. To further improve the control of the synthesis of nanoparticles with precisely tailored properties, a better understanding of the growth mechanism is essential.1−4 The most frequent methods for mechanistic studies are performed ex situ by sampling small aliquots of a reaction solution at different times, followed by reaction quenching, sample purification, and then characterization.5,6 However, the sampling process may significantly perturb the reaction, and important intermediate species may not be observed with such methods. As a result, it becomes critical to develop in situ techniques with noninvasive probes that can penetrate reaction solutions to observe nanocrystals and provide information that can help us evaluate physical parameters (such as size, shape, crystalline phase, etc.) of the nanocrystals. Time-resolved smallangle X-ray scattering (SAXS),7−10 wide-angle X-ray scattering (WAXS),11−13 X-ray absorption fine structure (XAFS) spectroscopy,14,15 transmission X-ray microscopy (TXM)16,17 with the use of intermediate-energy (10−20 keV) synchrotron Xrays, and in situ transmission electron microscopy (TEM)18−21 have been explored to study the growth of colloidal nanoparticles in liquid environment. However, the weak © 2012 American Chemical Society
penetration of these probes in liquid solutions requires small volumes inside of the in situ reactors that may cause problems such as insufficient diffusion of precursors and products and nonuniform temperature distribution. In addition, strong absorption of these X-rays and electrons in the reaction precursors and solvent molecules can induce unexpected side reactions. Herein, we report the use of a high-energy (70 keV) synchrotron X-ray beam that exhibits deep penetration into liquid solutions and weak absorption in reactants as a probe to monitor nanophase evolution involved in the synthesis of Ag nanocubes by continuously recording X-ray diffraction (XRD) patterns. A similar high-energy XRD technique with lower time resolutions has been used to study the growth of nanoparticles in high-pressure reactors containing supercritical fluids (e.g., TiO2 nanoparticles,22 CexZr1−xO2 nanoparticles,23 monoclinic zirconia nanocrystals,24 magnetite nanoparticles,25 ceria nanoparticles26), hydrothermal reactors containing aqueous solutions (e.g., nanostructured W/Mo oxides,27 TiO228), and open reactors (e.g., cobalt hydroxide nanoplates).29 Received: May 10, 2012 Published: May 11, 2012 11842
dx.doi.org/10.1021/jp304557p | J. Phys. Chem. C 2012, 116, 11842−11847
The Journal of Physical Chemistry C
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Article
EXPERIMENTAL METHODS AND DATA PROCESSING Synthesis of Colloidal Ag Nanocubes. All chemicals used were as received without any further purification. The synthesis of Ag nanocubes was carried out in a custom-made reaction vessel (Figures S1 and S2). Briefly, 0.3 mmol of dimethyl distearyl ammonium chloride (DDAC, >97%, Aldrich) was dissolved in a binary solvent including 8 mL of octyl ether (OE, 99%, Sigma-Aldrich) and 1 mL of oleylamine (OAm, approximate C18-content 80−90%, Acros Organics) in the reaction vessel under magnetic stirring. The solution was then heated to 260 °C with a heating mantle under a N2 atmosphere. After the temperature was stabilized at 260 °C, the X-ray beam shutter was opened for real-time probing of the nanocrystal growth. To capture the nucleation and growth events involved in the formation of nanoparticles, a continuous high-speed collection of XRD patterns was triggered. 1.0 mL of OAm solution of 0.2 M AgNO3 (99.99%, Strem Chemicals) was then quickly injected (at the rate of 32 mL/min) into the hot DDAC solution with a syringe pump (KDS200, KD Scientific Inc.). The mixing initiated the reactions instantaneously, and the time at which the AgNO3 precursor was injected was assigned as t = 0 s. 180 frames were first acquired, and each frame was associated with 1 s exposure (Figure S3a). After about 4 min, the data collection was switched to an intermittent mode in which only one XRD pattern with a 5 s exposure is acquired every minute (Figure S3b). The shutter was only opened during XRD pattern recording to minimize any possible influence of high-energy photons on the reactions. In a synthesis for ex situ characterization, aliquots (∼0.5 mL) were quickly taken out from the reaction vessel at various times (i.e.,
