Coarsening in sol-gel silica thin films

Autor: T. M. Harris, E. T. Knobbe
Rok vydání: 1996
Předmět:
Zdroj: Journal of Materials Science Letters. 15:132-133
ISSN: 1573-4811
0261-8028
DOI: 10.1007/bf00291446
Popis: Sol-gel derived films have been examined for potential use in a variety of applications, including chromatography columns, supported catalysts, gas separation membranes and chemical/biochemical sensors. In part, these systems rely on the controlled development of a porous microstructure (and concomitant large surface area) in the film. Retention of these characteristics during use is equally important. In a companion letter [1], the authors report the use of dye adsorption measurements for determining the porosity (more accurately, the surface area) of sol-gel silica thin films. The use of dye adsorption measurements to quantify the surface area of porous solids was pioneered by Giles and coworkers [2]. In principle, the amount of dye adsorbed on the surface of the solid will increase monotonically with time until equilibrium adsorption is attained. Also, as the concentration of the dye in solution is increased, the amount adsorbed will increase, at least up to the point at which the surface becomes covered with a monolayer of the dye. Interestingly, neither of these characteristics were observed in our experiments with sol-gel silica thin films. Discussion of this phenomenon and some of its implications are presented herein. The 'two-step' sol-gel processing scheme [3, 4] was utilized in this work. Sols were prepared with a water-to-tetraethoxysilane (TEOS) mole ratio of 2, an ethanol-to-TEOS mole ratio of 6, an HCI-toTEOS ratio of 0.005 and an ammonia-to-TEOS mole ratio of 0.015. Thin films were then made by dipcoating glass slides, employing a withdrawal rate of 12 crrdmin. The films were then dried in an oven (in air) at 115 °C. Other details of the preparation of the silica thin films are given in the companion letter [11. Dip-coated slides were immersed in a series of methylene blue (MB) solutions with concentrations ranging from 0 to 0.10 mM. The difference between the absorbance of MB in the initial solution and in a solution in which a slide was immersed was used to calculate the amount of dye adsorbed onto the deposit on the film. The length of time the slides were immersed in the dye solutions was varied; in some cases contact was maintained for several days. A plot of the amount of methylene blue adsorbed (in units of mmoles adsorbed per gram of sol-gel silica deposit) versus the concentration in the liquid phase is presented in Fig. 1. A 'reversal' in the
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