Abstract
Solutions derived by cosolvent-free (solventless) acid-catalyzed hydrolytic polycondensation of tetramethoxysilane (TMOS)–water and tetraethoxysilane (TEOS)–water binary systems were used to form silica films by dip-coating. Reactions in these solutions were examined by liquid-state 1H and 29Si nuclear magnetic resonance (NMR) spectroscopy, and the results were quantitatively analyzed based on a general reaction formula of tetraalkoxysilane deduced in this study. Transparent crack-free sintered silica films as thick as ∼500 nm were obtained from the TEOS-based solutions aged at 80 °C, and similar films were also prepared from solutions stored for 30 d at room temperature. Water molecules were nearly absent both in the TMOS and TEOS systems. Unreacted SiOH groups were depressed in the TMOS system whereas abundant in the TEOS system. These SiOH groups in the TEOS system are essential in producing uniform thin films as they render silica oligomers hydrophilic to increase affinity to glass substrates. Furthermore, they were inert and their slow polycondensation enables the extension of pot life. Films prepared from the TMOS system were non-uniform and cracked because the lack of residual SiOH groups impaired wettability to glass substrates and permitted the shrinkage of deposited liquid films before consolidation.
