Single Copper Nanoparticle Oxidation

Licentiate thesis, 2019

Cu nanoparticles are commonly used in microelectronic devices and as catalysts in, for example, methanol synthesis and methanol steam reforming reactions. However, Cu nanoparticles are prone to oxidation. During the oxidation process so-called Kirkendall voids often form due to different diffusion rates of oxygen and copper ions through the growing oxide. The growing void entirely transforms the nanoparticle structure, leading to fatal failure of microelectronic devices or to radically different catalytic properties. It is therefore of fundamental interest to gain deeper insight into the oxidation mechanism of nanoparticles to, for instance, understand under which conditions Kirkendall voids form. For this purpose, it is ideal to study the oxidation of Cu nanoparticles at the single particle level under close-to real application conditions since there is evidence that the grain structure significantly affects the oxidation mechanism and may accelerate or suppress the Kirkendall void formation.
 
Since only a handful of single particle studies concerning the oxidation of Cu nanoparticles exist to date, all but one by means of transmission electron microscopy (TEM), in this thesis I have developed an in situexperimental method for studying the oxidation of single Cu nanoparticles. It combines the structural information from TEM imaging with a non-invasive optical dark-field scattering spectroscopy method – plasmonic nanospectroscopy – that enables the tracking of oxidation kinetics in real time. In this way, I can minimize exposure of the particles to the electron beam in the TEM, and thus minimize the risk for beam-induced structural and chemical changes to the particles during the experiments.
 
Using this platform in combination with finite-difference time-domain electrodynamics simulations of models representing different stages during the oxidation, I was able to systematically analyze the single particle optical response measured in the experiments and thus shed light on the oxidation of the single Cu nanoparticles from a mechanistic perspective. As the first key result, we found a distinct evolution of the single particle dark-field scattering spectra of single Cu nanoparticles indicative of Kirkendall void formation. As the second key result, we identified a clear dependence of the induction time to the onset of Kirkendall void formation on the grain structure of the single nanoparticles, where an abundance of high-angle grain boundaries favors the coalescence of vacancies into one large void.