Structural Characterization of Mesoporous Thin Film Architectures: A Tutorial Overview.

Alberto Alvarez-Fernandez,Stefan Guldin,Giorgio Divitini,Alaric Taylor,Maximiliano J Fornerod,Barry Reid

doi:10.1021/acsami.9b17899

Abstract

Mesoporous thin film architectures are an important class of materials that exhibit unique properties, which include high surface area, versatile surface functionalization, and bicontinuous percolation paths through a broad library of pore arrangements on the 10 nm length scale. Although porosimetry of bulk materials via sorption techniques is common practice, the characterization of thin mesoporous films with small sample volumes remains a challenge. A range of techniques are geared toward providing information over pore morphology, pore size distribution, surface area and overall porosity, but none of them offers a holistic evaluation and results are at times inconsistent. In this work, we present a tutorial overview for the reliable structural characterization of mesoporous films. Three model samples with variable pore size and porosity prepared by block copolymer (BCP) coassembly serve for a rational comparison. Various techniques are assessed side-by-side, including scanning electron microscopy (SEM), atomic force microscopy (AFM), grazing incidence small-angle X-ray scattering (GISAXS), and ellipsometric porosimetry (EP). We critically discuss advantages and limitations of each technique and provide guidelines for reliable implementation.

Highlights

Over the past 20 years, ellipsometric porosimetry (EP) has emerged as sensitive and nondestructive physisorption technique that is capable of measuring structural characteristics in a optically transparent thin film geometry.[61−64] It provides a real-time measurement of the ellipsometric angles (Δ, Ψ), which can be related to the refractive index (RI) and film thickness as a function of relative pressure of a sorbent vapor within a sample chamber.[61,63]
To gain more quantitative insights of the porous structures, the power spectral density (PSD) function was calculated for each sample (Figure S1A)
We observed a slight increase in the center-to-center pore distance when decreasing the organic:inorganic ratio of the block copolymer (BCP), from Dc‐c(A20) = 23.2 ± 3.2 nm to Dc‐c(A10) = 26.8 ± 2.5 nm, which is in agreement with previous observations.[28]

Summary

■ INTRODUCTION

Mesoporous architectures with pore diameters in the range of 2−50 nm and film thicknesses up to 10 μm are promising components for a wide range of applications, such as gas and energy storage,[1−3] separation and purification membranes,[4−6] photovoltaics cells,[7−9] chemical-/biosensors[10−15] or optical coatings.[16−19] a plethora of different synthetic routes exist, including the random dense-packing of nanoparticles,[18,20] use of block copolymer (BCP) self-assembly with partial removal and optional backfilling[21−23] and the coassembly from combined solutions of inorganic precursors with organic structure-directing agents such as small molecule surfactants,[24] BCPs,[25−28] or colloids.[29]. Over the past 20 years, ellipsometric porosimetry (EP) has emerged as sensitive and nondestructive physisorption technique that is capable of measuring structural characteristics in a optically transparent thin film geometry.[61−64] It provides a real-time measurement of the ellipsometric angles (Δ, Ψ), which can be related to the refractive index (RI) and film thickness as a function of relative pressure of a sorbent vapor within a sample chamber.[61,63] Changes in RI are linked to the adsorption and gradual filling of the open pores with gas phase sorbent molecules, leading to a complete uptake by capillary condensation.[61] Analysis of the sample isotherm enables to determine an overall accessible porosity, pore size distribution, surface area and mechanical properties.[61−65] EP provides a number of distinct advantages in comparison to imaging or scattering techniques. Prior to EP measurements, samples were placed on a hot plate at 120 °C for 10 min to remove residual atmospheric water molecules inside the pores

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES