(158d) 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.