Characterization and preparation of transparent mesostructure silica thin films by spin coating method

Abstract

Transparent mesoporous silica thin films were successfully fabricated via spin coating method using precursor solutions of water/tetraethyl orthosilicate (TEOS) system with cetyltrimethylammonium bromide (CTAB) and nitric acid catalysis. The optimum conditions for the fabrication of films were as follows: 1) The molar ratios of precursor solution were TEOS/CTAB/H2O=3-9.5/1/85 in the pH range 2-3 adjusted by nitric acid. The precursor solution became transparent after aging for 2-3 h, and transparent thin films were obtained by spinning at 1000-5000 rpm. 2) The thickness of transparent thin films obtained by a single spin-coating was 0.5-1.2 μm. The transmittance to visible light and the refractive index of thin films were measured to be 97% and 1.44, while Vickers hardness and Young's modulus were 0.706 GPa and 32.2 GPa, respectively. The transparency of thin films was unchanged after drying at 373 K or firing at 723 K. 3) The characteristic diffraction peak of mesoporous structure at 2θ=2-3° in X-ray diffraction (XRD) diagrams was observed for all transparent thin films. The crystal phase of these films varied from lamella, hexagonal to cubic when compositions of TEOS/CTAB in precursor solutions were increased from 3-7, 7.5-8 to 9-9.5.