273406 Understanding the Effects of Synthesis and Treatment On Network Stability and Surface Chemistry of Silica
Porous silica nanoparticles have many unique features enabling a wide range of applications including drug delivery, imaging and catalysis. One of the most attractive features of silica for nano-applications is the ease of preparation and compatibility of the synthesis precursors with a variety of materials. It has recently been shown that freshly synthesized solid colloidal silica can be made porous by simple hydrothermal treatment resulting in etching, and thus porosity, with some control over final porosity based on the duration of treatment. The complex structure of silica, however, poses some challenges to the extension of this method to various systems. For example, experimental observations suggest that the mechanism of silica etching in water occurs through the breaking of siloxanes (Si-O-Si), and that the stability of these siloxanes is drastically altered by heat treatment in air or in vacuo. Furthermore, it has been shown that the surface of silica calcined in air at high temperatures could be fully rehydroxylated, without etching of the structure, by boiling in water. In order to fully exploit the potential of silica materials, a thorough understanding of both structural stability and surface chemistry is necessary.
The present study investigated the effects of synthesis, pretreatment and hydrothermal treatment on the porosity and surface chemistry of the silica nanoparticles with the overall goal of controlling these material properties. Silica nanoparticles were synthesized over a wide size range (6 – 500nm) and characterized via BET, TEM, and surface silanol quantification via probe molecule titration and UV-Vis spectroscopy. Calcination in air at 500 °C increased structural stability, making the silica resistant to etching in water as reflected in constant surface areas through multiple cycles of calcination and hydrothermal treatment. An interesting correlation between surface silanol concentration and apparent surface area, measured by both nitrogen and argon physisorption, was observed. Dehydroxylation of the sample resulted in a decrease of measured surface area, proportional to the reduction of surface silanol concentration. The effect of surface silanols on gas adsorption is currently being investigated via Monte Carlo computational studies.