Electron Microscopy of Copper Nanoparticle Growth
Authors: Elkjær Christian F. , van den Berg Roy , Gommes Cedric J , Chorkendorff Ib , Sehested Jens , de Jongh Petra E. , de Jong Krijn P. , Helveg S 
Published: December 2016

Understanding processes leading to the formation of nanoparticles is of utmost importance for tailoring their size, shape and spatial arrangements, which are crucial for the nanoparticles' use in heterogeneous catalysis. As the nanoparticle growth involve processes at or across atomic surfaces, observation made
in situ
at high‐spatial resolution would be beneficial for elucidating the nanoparticle growth mechanism. In recent years, transmission electron microscopy (TEM) has become a powerful technique for studying nanoparticles. With TEM, individual nanoparticles can be observed at atomic‐resolution and in reactive gas environments [1]. This capability opens up new possibilities for uncovering dynamics of nanoparticles immersed in gas environments. Here, we employ such
in situ
TEM capabilities to study the growth of Cu nanoparticles supported on SiO
2
by the reduction of a homogeneous precursor consisting of Cu phyllosilicate platelets (Fig. 1) [2, 3]. The homogeneous precursor represents an important class of industrial catalyst precursor material for which local TEM observations can directly be related to the catalyst at a macro‐scale.

By monitoring the individual nanoparticles over time, quantitative information about the nucleation time and size evolution of the Cu nanoparticles were obtained from TEM image series (Fig. 2). To employ such quantitative data in a kinetic description of the reduction process, it is imperative to reduce the influence of the electron beam on the process to a negligible level. To achieve that, we developed a low dose‐rate TEM imaging procedure in which several regions of the sample was monitored in parallel in a dose‐fragmentated way [1,4,5,6]. This strategy allows for a direct evaluating of the role of the electron dose‐rate, accumulated dose and dose history, which were all found to play a role on the growth process (Fig. 1C). Based on this evaluation, the role of the electron beam was quantitatively assessed and used to pinpoint illumination conditions that allowed the observation of the inherent thermal reduction process.

In view of this detailed analysis, quantitative data were extracted from the time‐resolved TEM image series to describe the thermally induced growth of the Cu nanoparticles supported on SiO
2
(Fig 1B). By comparing the quantitative TEM data of the process with kinetic models, it was found that the growth process was best characterized as an autocatalytic reaction with either diffusion‐ or reaction‐limited growth of the nanoparticles. This finding is significant because the autocatalytic reaction limits probability for secondary nucleation and because the platelike precursor structure restricts the diffusion. This describes a way to synthesize nanoparticles with well‐defined sizes, which in turn offers a more stable catalyst[7]. In this way,
in situ
observations made by electron microscopy provide mechanistic and kinetic insights that can guide the formation of metallic nanoparticles for catalytic applications in a rational way.