Investigating Silica-Based Mesoporous Films Using Ellipsometric Porosimetry

Ellipsometric porosimetry can be used to probe the internal wettability of silica-based mesoporous films. The wettability of nanostructured surfaces is critical for applications such as self-cleaning and smart windows, antireflective coatings, membranes, and gas-sensing devices.

However, while the study of wettability and the determination of contact angle (denoted by θ) is regularly performed on planar surfaces with simple experimental setups, the resultant macroscopic wettability may not agree with the intrinsic (microscopic) wetting behavior of nanostructured materials.

Ellipsometric porosimetry (carried out using the PS-2000 Ellipsometric Porosimeter series from Semilab, Figure 1.) is one of the most powerful characterization methods for mesoporous thin films.

It delivers reliable pore size, porosity, specific surface area, and Young's modulus information in a completely optical, non-destructive manner for both thin film mono- and multilayers.

Figure 1. PS-2000. Image Credit: Semilab Semiconductor Physics Laboratory

Metrology

In this study, ellipsometric porosimetry (EP) was used to measure how the optical properties of a thin film change when solvent vapor is adsorbed into and desorbed from its pores, allowing a series of film thicknesses and refractive indices to be obtained.

First the thin film properties of SiO₂ samples functionalized for various times were determined by spectroscopic ellipsometry (SE) spectra at P/P₀ (water)=0. (see Figure 2. and 3.)

Figure 2. SE system’s mechanism. Image Credit: Semilab Semiconductor Physics Laboratory

Figure 3. SE parameters (Ψ, Δ) recorded during a water adsorption EP cycle on the non-functionalized silica surface. Image Credit: Semilab Semiconductor Physics Laboratory

Use Case

Ellipsometric porosimetry was employed to study the internal wettability of methyl-functionalized silica materials. The synthesis route is detailed on Figure 4.

Figure 4. Sol-gel synthesis route with TMCS/hexane methyl-functionalized mesoporous silica films. A: polymer-silica hybrid sol, B: polymer-silica hybrid thin film, C: as-calcined mesoporous silica thin film, D: methyl-functionalized mesoporous silica thin film. Image Credit: Semilab Semiconductor Physics Laboratory

Methyl-functionalization of the silica surface results in a delay in capillary condensation in the acquired volume-adsorbed isotherms. However, this effect was considerably more significant when considering the two more polar adsorptives.

Comparing functionalized and non-functionalized silica surfaces allowed the internal contact angle values to be calculated for all four tested adsorptive liquids (water, methanol, toluene, cyclohexane) – see Figure 5 and 6.

Figure 5. B, illustrates the pore size distributions (PSD) calculated from the isotherms measured with toluene shown on A, assuming 0° internal contact angle. C, demonstrates the PSD with fitted contact angles. Image Credit: Semilab Semiconductor Physics Laboratory

Figure 6 displays the resultant internal contact angles. The effect of both surface energy and adsorbate polarity may be observed. Thereby a powerful method was presented for systematically comparing the internal wettability of a materials library with a wide range of surface energies towards liquids with a broad spectrum of polarities.

Figure 6. Internal contact angles of the TMCS-functionalized silica surfaces for the four investigated liquids as EP adsorbates. Image Credit: Semilab Semiconductor Physics Laboratory

Materials & Structures

Porous thin films ( TiO₂, SiO₂, WO₃, ZnO), nanoparticles, low k-, mesoporous- & microporous materials.

For more information you can visit the following related SEMILAB publication:

Máté Füredi et al., “Internal wettability investigation of mesoporous silica by ellipsometric porosimetry” in Science Direct Volume 768, 1 March 2023, doi: 10.1016/j.tsf.2023.139683.

Further EP related SEMILAB publications:

1. P. Márton, et al., „Model investigation of a consecutive dye uptake and release process by using a bilayered chitosan-mesoporous silica nanocoating system” in International Journal of Biological Macromolecules, 2026, Volume 355, 151432, ISSN 0141-8130, 14 March 2026, DOI: 10.1016/j.ijbiomac.2026.151432
2. E. Albert, et al.,”Experimental and Computational Synthesis of TiO2 Sol–Gel Coatings” in Langmuir 2025 (ACS publication), Volume 41, Issue 1, 704–718, 2 January 2025, DOI: https://doi.org/10.1021/acs.langmuir.4c03959
3. P. Márton, et al., ” Chitosan nanocoatings N-acylated with decanoic anhydride: Hydrophobic, hygroscopic and structural properties” in Carbohydrate Polymers (Elsevier), Volume 343, 1 November 2024, 122480, DOI: https://doi.org/10.1016/j.carbpol.2024.122480
4. M. Füredi, et. al.,” Beyond the Meso/Macroporous Boundary: Extending Capillary Condensation-Based Pore Size Characterization in Thin Films Through Tailored Adsorptives” in J.Phys.Chem. Lett. 2024, 15 (5), 1420-1427, 30 January 2024, DOI: 10.1021/acs.jpclett.3c03442
5. L. Kócs et al., “Ammonia-vapour-induced two-layer transformation of mesoporous silica coatings on various substrates” in Science Direct Volume 192, Oct. 2021, DOI: 10.1016/j.vacuum.2021.110415
6. A. Ábrahám, et. al., ”Durability of microporous hybrid silica coatings: Optical and wetting properties” in Science Direct Volume 699, 1 April 2020, DOI: https://doi.org/10.1016/j.tsf.2020.137914
7. B. Tegze, et al., “Photoinduced processes of adsorbed and associated dye molecules in mesoporous titania coatings” in Science Direct Volume 167, August 2019, DOI: 10.1016/j.dyepig.2019.04.017
8. L. Kócs et al., “Silica Sol-gel Coatings with Improved Light Transmittance and Stability” in Semantic Scholar, DOI: 10.3311/PPCH.10550
9. C. Robertson et al., “Surface modification and porosimetry of vertically aligned hexagonal mesoporous silica films” in Publishing Issue 114, 2016, DOI: 10.1039/C6RA23059H
10. E. Albert, et al., “Antibacterial properties of Ag–TiO₂ composite sol–gel coatings” in Publishing Issue 73, 2015, DOI: 10.1039/C5RA05990A
11. E. Albert, et al., “Introducing nanoscaled surface morphology and percolation barrier network into mesoporous silica coatings” in Publishing Issue 74, 2015, DOI: 10.1039/C5RA09357K

This information has been sourced, reviewed and adapted from materials provided by Semilab Semiconductor Physics Laboratory.

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