Balls, beams and blocks: In situ observation of colloidal particles in confinement and under electron irradiation

We studied the dynamic behavior of colloidal particles – approximately between 10 and 1000 nanometers – in real space on the single particle level using a microscope. In particular, we studied the dynamic behavior of particles that were not freely diffusing but instead were restricted in their motion.
From the (collective) behavior of colloidal particles, it is possible to extract particle properties and interparticle interaction potentials. Such analyses were performed on silica spheres that repel one another over large distances. Additionally, colloidal particles were synthesized that were composed of a different kind of silica by incorporation of organic groups containing a diamine moiety. By using such organically modified silica combined with etching in a solution of hydrofluoric acid, it was possible to tune the particle charge – even reversing its sign – and synthesize yolk-shell particles: a sphere enclosed by a larger shell.
The dynamics of these captured particles were investigated using (optical) confocal microscopy. The electrostatic repulsion between the particle and the shell was found to be stronger than expected. Also, the particle diffusivity was decreased more than expected based on hydrodynamic coupling with the shell.
Silica yolk-shell particles dispersed in water were also investigated using liquid cell electron microscopy. However, the electron beam that is used to produce images of the particles caused drastic changes in the particles. Upon exposure to the electron beam, the particles initially increased in size before steadily shrinking. Both of these processes were explained by a mechanism of beam induced breaking of chemical bonds in the silica structure.
Similar beam induced changes were observed for nanocrystals of magnesium oxide. In the presence of water vapor, irradiation caused these oxide crystals to convert to the hydroxide, Mg(OH)2, with the reaction initiating from the particle surface. Owing to the high spatial resolution of the electron microscope, the reaction front was observed to travel inwards at a constant rate, gradually consuming the oxide core and leaving a hydroxide coating.