Anisotropic Shape Changes of Silica Nanoparticles Induced in Liquid with Scanning Transmission Electron Microscopy

Abstract

Liquid-phase transmission electron microscopy (TEM) is used for in-situ imaging of nanoscale processes taking place in liquid, such as the evolution of nanoparticles during synthesis or structural changes of nanomaterials in liquid environment. Here, it is shown that the focused electron beam of scanning TEM (STEM) brings about the dissolution of silica nanoparticles in water by a gradual reduction of their sizes, and that silica redeposites at the sides of the nanoparticles in the scanning direction of the electron beam, such that elongated nanoparticles are formed. Nanoparticles with an elongation in a different direction are obtained simply by changing the scan direction. Material is expelled from the center of the nanoparticles at higher electron dose, leading to the formation of doughnut-shaped objects. Nanoparticles assembled in an aggregate gradually fuse, and the electron beam exposed section of the aggregate reduces in size and is elongated. Under TEM conditions with a stationary electron beam, the nanoparticles dissolve but do not elongate. The observed phenomena are important to consider when conducting liquid-phase STEM experiments on silica-based materials and may find future application for controlled anisotropic manipulation of the size and the shape of nanoparticles in liquid.

Nanoscale Imaging and Stabilization of Silica Nanospheres in Liquid Phase Transmission Electron Microscopy

“…As seen in Figure 1, silica particles vary in size, and the majority of the particles have a diameter between 60 and 100 nm, but with a fraction of 20-30 nm diameter particles (see also Figure S1, Supporting Information). As was previously reported [35] and also visible in Figure 1c, the scanning of the electron beam introduced elongation in the scanning direction and shrinkage of particles.…”

“…Images showing silica particles before each change in scanning direction and movies of the whole experiment are available in Figure S2 and Movies S1 and S2 in the Supporting Information. In accordance with previous research, [35] the particles tended to elongate in the scanning direction of the beam, while shrinking in the direction perpendicular to that ( Figure S2, Supporting Information). The material removal rate in the perpendicular direction was found to be higher than the growth of the particles in the scanning direction, resulting in a net loss of volume.…”

“…To understand the mechanism of silica degradation under LP‐TEM conditions, two important phenomena observed in this study will be considered: detrimental influence of reducing radicals formed by the electron beam induced water radiolysis and the stabilizing effect of radical scavengers. As suggested previously, mass loss occurs primarily as a result of dissolution of silica, which in water is predominantly through formation of silicic acid or Si(OH) 4 species. The formation of silicic acid requires hydroxylation of the silica, which usually proceeds through the opening/hydrolysis of siloxane bonds by H 2 O .…”

“…One of the possible parameters relevant for silica degradation observed in Figure 1c is the solid structure of the silica itself, in particular the porosity and the degree of hydroxylation of both the surface and bulk of the particles. [35] Lowering the degree of silica condensation could result in increasing susceptibility toward excitation and destabilization by the electron beam. This would facilitate reactions with water, leading to the formation of soluble Si(OH) 4 species thought to be responsible for the deformation and dissolution of the particles.…”

“…[31] This destabilizing effect is also seen for amorphous SiO 2 , an important oxide in many nanoscale applications, [32][33][34] which although stable in vacuum, is severely destabilized by electron beam induced chemistry in water. [35,36] Specifically, it was shown that upon imaging with a scanning electron beam, silica nanospheres tended to elongate in the scanning direction of the beam and shrink significantly over time. To date however, most studies in LP-TEM focused on metallic nanoparticles, resulting in a limited understanding of oxide behavior under these conditions.To enable LP-TEM studies free from adverse electron beam effects, on not only amorphous silica, but also other metals and…”