In Situ Study of the Wet Chemical Etching of SiO2 and Nanoparticle@SiO2 Core-Shell Nanospheres

The recent development of liquid cell (scanning) transmission electron microscopy (LC-(S)TEM) has opened the unique possibility of studying the chemical behavior of nanomaterials down to the nanoscale in a liquid environment. Here, we show that the chemically induced etching of three different types of silica-based silica nanoparticles can be reliably studied at the single particle level using LC-(S)TEM with a negligible effect of the electron beam, and we demonstrate this method by successfully monitoring the formation of silica-based heterogeneous yolk-shell nanostructures. By scrutinizing the influence of electron beam irradiation, we show that the cumulative electron dose on the imaging area plays a crucial role in the observed damage and needs to be considered during experimental design. Monte-Carlo simulations of the electron trajectories during LC-(S)TEM experiments allowed us to relate the cumulative electron dose to the deposited energy on the particles, which was found to significantly alter the silica network under imaging conditions of nanoparticles. We used these optimized LC-(S)TEM imaging conditions to systematically characterize the wet etching of silica and metal(oxide)-silica core-shell nanoparticles with cores of gold and iron oxide, which are representative of many other core-silica-shell systems. The LC-(S)TEM method reliably reproduced the etching patterns of Stober, water-in-oil reverse microemulsion (WORM), and amino acid-catalyzed silica particles that were reported before in the literature. Furthermore, we directly visualized the formation of yolk-shell structures from the wet etching of Au@Stober silica and Fe3O4@WORM silica core-shell nanospheres.

Automated summary

The recent advancement of liquid cell (scanning) transmission electron microscopy (LC-(S)TEM) enables the study of nanomaterials at the nanoscale in a liquid environment, showing the chemically induced etching of silica nanoparticles can be reliably studied at the single particle level with negligible electron beam effects. Monte-Carlo simulations of electron trajectories during LC-(S)TEM experiments can relate the cumulative electron dose to deposited energy on particles, significantly altering the silica network under imaging conditions. LC-(S)TEM imaging conditions were optimized to systematically characterize the wet etching of silica and metal(oxide)-silica core-shell nanoparticles, reproducing etching patterns of different types of silica particles reported in literature and directly visualizing the formation of yolk-shell structures from wet etching of core-shell nanospheres.