Low‐dose liquid cell electron microscopy investigation of the complex etching mechanism of rod‐shaped silica colloids

Abstract

Understanding the chemical structure of rod-shaped silica colloidal particles is attainable by investigating their etching mechanism in solution. Liquid Cell (Scanning) Transmission Electron Microscopy (LC-(S)TEM) is a promising technique through which the etching of these particles can be observed in real time, and at the single particle level, without possible deformations induced by the surface tension of dried particles. However, the presence of high energy electrons, and the different geometry in LC experiments may alter the conditions of in situ experiments compared to their ex situ counterparts. Here we present a controlled low-dose LC-STEM study of the basic etching process of micron-sized silica rods that are immobilized on the SiN window of a liquid cell. The results show that using low-dose imaging conditions, combined with a low accumulated electron dose, and optimized flow rates of solutions allow for investigation of the chemical etching mechanism of silica colloidal particles using the LC-(S)TEM technique with negligible effects of the electron beam. A comparison of ex situ etching experiments with LC-STEM observations show that the LC geometry can play a crucial role in LC-STEM experiments where the diffusion of the etching particles is important, which should be considered during the interpretations of LC-STEM results.

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

“…Maintaining a flow of basic solution has been shown to prevent base depletion during wet etching of silica in liquid cell experiments. 48 The use of buffers could also help maintain the pH inside the cell during etching experiments. However, this function is already fulfilled by the constant flow set-up.…”

“…At such high electron dose rates, the scanning induced the deformation of silica particles in the scanning direction, as previously reported for silica spheres and colloidal silica rods under intense STEM illumination. 31 , 48 , 49 Although this process is still not well understood, silica plasticity increases under electron irradiation even at room temperature, 50 and thus, we might be observing the effects of the intense electric fields created by secondary electron ejection from the surfaces in the liquid cell. 51 To summarize, our results indicate that the expansion process of silica particles upon electron beam irradiation in the presence of water appears to be completely driven by the cumulative electron dose rate on the particle and that the extent of such expansion depends on the media in which the particles are immersed in.…”

“…The constant flow of the base ensured that the cell was always under a flow of basic solution and in contact with a constantly renewed reservoir of the base. Maintaining a flow of basic solution has been shown to prevent base depletion during wet etching of silica in liquid cell experiments . The use of buffers could also help maintain the pH inside the cell during etching experiments.…”

Extending in situ liquid phase transmission electron microscopy to etching of silica and the swelling-collapse of temperature sensitive polymer microgels

“…Maintaining a flow of basic solution has been shown to prevent base depletion during wet etching of silica in liquid cell experiments. 109 The use of buffers could also help maintaining the pH inside the cell during etching experiments. However, this function is already fulfilled by the constant flow set-up.…”

“…At such high electron dose rates the scanning induced the deformation of silica particles in the scanning direction, as was previously reported for silica spheres and colloidal silica rods under intense STEM illumination. 106,109,110 Although this process is still not well understood, silica plasticity increases under electron irradiation even at room temperature, 111 and thus we might be observing the effects of the intense electric fields created by secondary electron ejection from the surfaces in the liquid cell. 112 To summarize, our results indicate that the expansion process of silica particles upon electron beam irradiation in presence of water appears to be completely driven by the cumulative electron dose rate on the particle, and that the extent of such expansion depends on the media in which the particles are immersed in.…”