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A facile approach for the synthesis of monodisperse gold nanoparticles with radii in the range of 7 to 20 nm is presented. Starting from monodisperse seeds with radii of 7 nm, produced in the first step, the addition of a defined amount of additional precursor material permits distinct size regulation and the realization of predicted nanoparticle sizes. These information were derived from ex- and in situ investigations by comprehensive small angle X-ray scattering (SAXS), X-ray absorption near edge structure (XANES) and UV-Vis data to obtain information on the physicochemical mechanisms. The obtained mechanisms can be transferred to other seeded growth processes. Compared to similar approaches, the presented synthesis route circumvents the use of different reducing or stabilizing agents. The size of resulting nanoparticles can be varied over a large size range presented for the first time without a measurable change in the shape, polydispersity or surface chemistry. Thus, the resulting nanoparticles are ideal candidates for size dependence investigations.
Gold nanoparticles (AuNP) were prepared by the homogeneous mixing of continuous flows of an aqueous tetrachloroauric acid solution and a sodium borohydride solution applying a microstructured static mixer. The online characterization and screening of this fast process (~2 s) was enabled by coupling a micromixer operating in continuous-flow mode with a conventional in-house small angle X-ray scattering (SAXS) setup. This online characterization technique enables the time-resolved investigation of the growth process of the nanoparticles from an average radius of ca. 0.8 nm to about 2 nm. To the best of our knowledge, this is the first demonstration of a continuous-flow SAXS setup for time-resolved studies of nanoparticle formation mechanisms that does not require the use of synchrotron facilities. In combination with X-ray absorption near edge structure microscopy, scanning electron microscopy, and UV-vis spectroscopy the obtained data allow the deduction of a two-step mechanism of gold nanoparticle formation. The first step is a rapid conversion of the ionic gold precursor into metallic gold nuclei, followed by particle growth via coalescence of smaller entities. Consequently it could be shown that the studied synthesis serves as a model system for growth driven only by coalescence processes.
The formation mechanisms of silver nanoparticles using aqueous silver perchlorate solutions as precursors and sodium borohydride as reducing agent were investigated based on time-resolved in situ experiments. This contribution addresses two important issues in colloidal science: (i) differences and analogies between growth processes of different metals such as gold and silver and (ii) the influence of a steric stabilizing agent on the growth process. The results reveal that a growth due to coalescence is a fundamental growth principle if the monomer-supplying chemical reaction is faster than the actual particle formation.
Size-controlled synthesis of colloidal silver nanoparticles based on mechanistic understanding
(2013)
Metal nanoparticles have attracted much attention due to their unique properties. Size control provides an effective key to an accurate adjustment of colloidal properties. The common approach to size control is testing different sets of parameters via trial and error. The actual particle growth mechanisms, and in particular the influences of synthesis parameters on the growth process, remain a black box. As a result, precise size control is rarely achieved for most metal nanoparticles. This contribution presents an approach to size control that is based on mechanistic knowledge. It is exemplified for a common silver nanoparticle synthesis, namely, the reduction of AgClO4 with NaBH4. Conducting this approach allowed a well-directed modification of this synthesis that enables, for the first time, the size-controlled production of silver nanoparticles 4–8 nm in radius without addition of any stabilization agent.
Although metallic nanoparticles play an important role in the area of nanotechnology, a coherent mechanistic explanation for the evolution of the particles during their chemical synthesis has not yet been provided in many cases. To gain a profound understanding of the growth mechanism of colloidal nanoparticles, new approaches using Small Angle X-Ray Scattering (SAXS) combined with X-ray absorption near-edge structure (XANES) are presented. This combination allows for insights into two prominent syntheses routes of gold nanoparticles (GNP): The 'slow' reaction using sodium citrate (30-90 min) as a reducing agent and the 'fast' reaction employing NaBH4 (within few seconds). In the first case data derived with the coupled XANES and SAXS suggests a four-step particle formation mechanism. For the second system a time resolution in the order of 100-200 ms was achieved by coupling a common laboratory SAXS instrument with a microstructured mixer, which allows data acquisition in a continuous-flow mode. The results indicate a coalescence driven growth process. Based on the capabilities to deduce the size, number and polydispersity of the particles, the results of both methods enable the development of mechanistic schemes explaining the different phases of particle formation and growth, thus providing a basis for improved control over the synthesis processes.
Efficient water electrolysis requires electrode coatings with high catalytic activity. Platinum efficiently catalyzes the hydrogen evolution reaction in acidic environments, but is a rare and expensive metal. The activity achieved per metal atom can be increased if small Pt particles are dispersed onto electrically conductive, highly accessible and stable support materials.
However, the addition of Nafion, a typical binder material used in the manufacture of electrode coatings, can decrease catalytic activity by the blocking of pores and active surface sites. A new approach is reported for the direct synthesis of highly active Nafion-free Pt/C catalyst films consisting of small Pt nanoparticles supported in size-controlled mesopores of a conductive carbon film. The synthesis relies on the co-deposition of suitable Pt and C precursors in the presence of polymer micelles, which act as pore templates. Subsequent carbonization in an inert atmosphere produces porous catalyst films with controlled film thickness, pore size and particle size. The catalysts clearly outperform all Nafion-based Pt/C catalysts reported in the literature, particularly at high current densities.