Infrared spectroscopic analysis of siloxane network modification of mesoporous silica film by silylation and cesium doping

Effects of pore surface silylation and cesium doping on the siloxane bonding structure of mesoporous silica (MPS) film were analyzed using infrared spectroscopy. MPS film was derived by sol-gel method and self-assembling technology of non-ionic surfactant template. Cesium (Cs) catalyst was doped to the precursor solution with the amounts of 0, 5, and 15 wt-ppm. Vapor phase silylation was performed by transporting tetramethylcyclotetrasiloxane (TMCTS) or hexamethyldisilazane (HMDS). From dispersion analysis of the infrared spectrum, transverse optical (TO) function and longitudinal optical (LO) function of silica skeleton, which concern the ionic vibration of siloxane bonding at ~ 1000-1300 cm−1, were determined. In terms of LO-TO splitting width of the disorder-induced oxygen vibration (AS2) mode, siloxane network connectivity was discussed. Oscillator strength of ionic vibration was also calculated quantitatively. It was found that TMCTS treated MPS had highly cross linked siloxane network with increased oscillator strength compared to those of HMDS treated MPS. Cs doping promoted siloxane network connectivity, while the oscillator strength was negligibly changed. The connectivity and oscillator strength were almost equal to those of 1000 °C annealed gel silica. Mechanical enhancement associated with the TMCTS treatment and Cs doping was presumably due to increased degree of the siloxane network.