Cosolvent-free sol–gel dip-coating of silica films from tetraalkoxysilane–water binary systems: precursor solutions of long pot life and their characterization by nuclear magnetic resonance spectroscopy

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 dipcoating. Reactions in these solutions were examined by liquid-state 1 H and 29 Si 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 ³500 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.

Current status of sol–gel processing of glasses, ceramics, and organic–inorganic hybrids: a brief review

“…12) The second approach has been employed in solventless TEOSwater binary systems and achieved a pot life of 30 days. 10) The combination of the first and second approaches has enabled to maintain the viscosity of alkoxide-based coating solutions of TiO 2 films for 37 days. 13),14) Dilution combined with the addition of a chelating agent (ethylene glycol) has extended the pot life of alkoxide and acetate-based precursor solutions of leadzirconate-titanate (PZT) ferroelectric films to 1 year.…”

“…( 1) and ( 2) have not been derived. On the assumption that the resulting oligomers have the same average composition and the coordination numbers of M and O (n and 2, respectively) do not change, a general equation at n = 4 can be expressed as 10) MðORÞ…”