Facile and adaptable synthesis method of mesostructured silica thin films

Abstract

By employing high speed spin coating for the film formation and high temperature (80 °C) drying and aging, we developed a synthesis procedure for mesostructured silica thin films that can reproducibly produce up to eight different pore morphologies, including a rhombohedral (Rm), a centered rectangular (c2mm), two wormlike, two curved lamellar, and two lamellar structures. The mesostructured films were characterized by low angle X-ray diffraction and transmission electron microscopy, and the rhombohedral and centered rectangular by N₂ adsorption in addition. The two measures we used in our procedure are believed to effectively quench the hydrolysis/condensation reactions of silica species after the coating sol–gel solution is cast into thin films, so that the mesostructures in the films are formed through the liquid-crystal mechanism. In this way, the final mesostructure is determined by the size and size distribution of the silica species, as probed by ²⁹Si-NMR spectroscopy, which are determined by the water and/or HCl contents in the coating solutions. We have been able to synthesize mesostructured films with large areas (up to 4 inch in diameter) and on various substrates (ITO, FTO, Si wafer, Pt, Au, and glass) without inducing any changes of the mesostructures. The mesostructured films grown on conductive substrates could serve as templates to grow electrochemically nanostructures of various materials that replicate the pore structures. The metal replicas provide deeper insights into the structures of the mesostructured films by revealing some features such as grain boundaries that are not obvious from the analyses of the mesoporous silica films only.