Sol-Gel-Derived Silica Films with Tailored Microstructures for Applications Requiring Organic Dyes
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r3/rD,
(1)
where r = cluster size and D = mass fractal dimension (0 < D < 3). Second, for D > 1.5, the tendency for mass fractal objects to interpenetrate decreases with increasing size, preserving the porosity between clusters as the film is formed. Aging a sol allows the inorganic clusters to grow larger, and aging (at intermediate to basic pH) also causes the clusters to coarsen and become stiffer [13], so a film from an aged sol will have 1) more porous clusters, 2) less interpenetration of clusters as they are concentrated, and 3) less collapse of the clusters due to the capillary forces exerted during drying [8-10]. There are two complications to this simple method of varying film porosity by varying the age of the sol. First, the tendency for fractal objects to interpenetrate is a function of D as well as r. 115 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
The tendency to interpenetrate is inversely related to the mean number of intersections M1 ,2 of two mass fractal objects of size r and mass fractal dimension D placed in the same region of space [12]: M 1,2 ,, r (2D -3).
(2)
According to Equation 2, for D > 1.5, M1 ,2 increases with both increasing r and increasing D, leading to a reduced tendency for interpenetration. However, Equation 1 indicates that the porosity of individual clusters increases with decreasing D. To maximize porosity, an intermediate value of D is required that balances cluster porosity and cluster-cluster interpenetration: if D is too large, individual clusters are not very porous, whereas if D is too small, cluster interpenetration reduces the porosity between clusters. We will show that D is a function of the sequence and pH of the sol preparation steps. The second complication is that small inorganic clusters can "fill-in" the pores of the deposited film, masking the porosity created by aging a sol. EXPERIMENTAL A two- or three-step acid/base-catalyzed process was used to prepare sols identified as B2 [10, 14-15], AAB [11], or. AAB(1/5), respectively. The first step was the same for all of the sols: tetraethoxysilane (TEOS), ethanol, water and HCI were mixed in the molar ratio 1:3.8:1.0:0.007, refluxed at 60*C for 90 ramn and cooled to room temperature [15]. This solution, referred to as stock solution, was used immediately or stored in a freezer at -20°C. For B2 the second hydrolysis step consisted of adding an aqueous solution of 0.05 M NH4 OH in additional ethanol, resulting in a final molar ratio of 1 TEOS:48 ethanol:3.7 H2 0:0.007 HCI:0.002 NH 4 OH, and a final sol pH of 5.5 as estimated using colorimetric pH indicator strips (EM Science). For AAB the second step consisted of adding 1 M HCI diluted in ethanol, resulting in a H2 0/Si ratio = 2.5, and refluxing at 60'C for 60 min. The third step consisted of adding an aqueous solution of 2 M NH4 OH diluted in ethanol, resulting in a final molar ratio of 1:48:3.7:0.028:0.05. For AAB(1/5) sols, the second step consisted of adding HCl, H2 0 and ethanol and refluxing for 1 or 4 h, bringing the mola
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