Carbon dioxide mediated synthesis of mesoporous silica films: Tuning properties using pressure

Mesoporous silica films were synthesized by the selective condensation of tetraethylorthosilicate (TEOS) within preformed amphiphilic templates using carbon dioxide as a carrier for the precursor. Blends of poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (Pluronic F108) and poly(p-hydroxystyrene) (PHOSt) containing p-toluenesulfonic acid were exposed to gaseous or supercritical fluid solutions of CO2, TEOS, and H2O at 60 degrees C. The subsequent sorption within the polymeric template delivered the precursors for selective reaction in the hydrophilic domains. Mesoporous silica films were formed by the removal of the template by calcination at 450 degrees C. The physical properties of mesoporous silica were tailored through control of the CO2 pressure during the reaction. The pore size increased slightly (1.6-1.81 nm) as the CO2 pressure was increased to 80 bar. A small increase in pressure resulted in a significant expansion of the pore size near 84 bar to nearly 4 nm pore radii. In addition to tuning the mesopore size, increasing the CO2 pressure during synthesis led to a decrease in the apparent reaction rate and subsequently an increase in the film porosity. Moreover, the apparent accessibility of the mesopores to small molecules as evidenced by the lack of capillary condensation of water and toluene was tuned by the CO2 pressure. Synthesis very near the critical pressure Of CO2 resulted in a small fraction of the pores being accessible. Thus, synthesis of mesoporous films using CO2 appears to be a promising route to limit the contamination of low-k dielectric materials for microelectronics applications.