A new approach for preparation of free-standing nano-porous SiO2 films with a large area

Free-standing nano-porous SiO 2 films with high porosity on the scale of hundreds of microns were obtained by combing a parting agent method and layer-by-layer assembly method where betaine was employed as soluble layer and PAMS (poly-α-methylstyrene) was used as protective and thermal degradation sacrificial layer, respectively. The surface morphology, roughness, chemical composition and physical properties of the films were studied in this work. The thermal degradation property of PAMS and its effect on the preparation of free-standing film were also analyzed. The results indicated that the PAMS effectively protected the SiO 2 films from damage during the detachment and transfer processes, and it can be completely removed after the film being calcined at 330 °C for 4 h. The thickness of the SiO 2 films can be adjusted between 534 and 297 nm. Before calcination, the SiO 2 film was hydrophobic and the process of dissolving betaine would not damage its structure nor influence its smoothness. After being calcined, the SiO 2 film became hydrophilic, the refractive index increased from 1.19 to 1.20, and the porosity decreased from 56 to 53 %; the calcination process contributed to progress condensation of the polysiloxane network and removed organic groups from silica film. Employing PAMS as protective and thermal degradation sacrificial layer provides a new approach for preparation of other large-area free-standing films. Graphical Abstract