Tuned optical transmittance in single-step-derived silica aerogels through pH-controlled microstructure

Abstract

We have systematically studied the relationship between synthesis pH and morphological and optical properties of silica aerogels. We have determined through SEM and BET that there is a systematic correlation between the pH of the initial silica solution and the surface area, porosity, and pore size of the resulting aerogel. We find that optical transmittance, particularly its wavelength dependence (dispersion), is strongly governed by the microstructure and, therefore, synthesis pH. We have determined that the microstructure of the aerogels fall into three broad categories: monostructural, which is characterized by repeating elongated microstructural features; fractal, which shows a distinctive structure that is emulated on multiple length scales; and isotropic, which is characterized by having no distinct features in its microstructure. Simply by controlling the pH of the synthesis environment, we can tune the optical properties of silica aerogels through pH controlled the microstructural modification. We have found that pH = 1-5 gives high dispersion, pH = 6-7 results in low transmittance and low dispersion, pH = 8 shows the highest transmittance with the lowest dispersion, and pH = 9-10 transitions back to lower transmittance and higher dispersion.

Physicochemical properties of ambient pressure dried surface modified silica aerogels: effect of pH variation

“…In the visible light spectra, scattering of the light increases as the inner surface of the pores of the prepared silica aerogels are more transparent. The light scattering in the prepared aerogels is may due to the divergence in the values of refractive indexes in between the silica network and inner pore of the gel which acts as scattering centers [20]. Samples with pH values 4, 6 and 7 shows relatively low transmittance whereas the sample with pH value 5 expresses the greater transmittance as shown in Fig.…”

“…There is variation in surface area of silica aerogel with pH value such that at natural condition surface area is less as compared to pH at 4. At pH 7 (neutral condition) condensation process dominates the hydrolysis process the molecule of precursors aggregates which causes density upsurge and hence the surface area is less [20]. As pH value increase, the surface area of silica decrease, pore radius obtained 19-21 Å, the variation of surface area, pore radius pore volume with respect to pH are summarized in Table 1.…”

“…As the pH of the silica sol increased from 5 to 7 the time of gelation increases. As pH is less than or equal to 5 means under acid-catalyzed conditions are in the favor of hydrolysis reactions that limit condensation reactions [18,20]. Because of this clustering of silica agglomerate at the quicker time comparatively which helps to form a porous silica network of threedimensional in a very short time.…”

“…He reported that by varying the synthesis conditions and parameters structure of the porous materials can be tailored. Twej et al [20] studied the correlation of pH with optical, morphological behaviour of aerogel samples. They reported that the pH variation can be used to control the optical parameters of silica aerogels.…”

Effect of Modified Silica Materials on Polyvinyl Chloride (PVC) Substrates to Obtain Transparent and Hydrophobic Hybrid Coatings

“…In this way, the networks were branched more locally, leading to narrow pore sizes and small pore volumes. Thus, dense networks were created, and the coating surface presented a low transparency [34].…”