February
Picture of the Month - February 2024
Achieving and Understanding Large Mesopores with PEO-b-PHA Block Copolymers
Mesoporous (2 – 50 nm in diameter) metal oxides are promising model materials in several fields of application such as heterogeneous catalysis, coatings, solar cells, and sensors due to their high and readily accessible surface area. Macroscopic properties like conductivity, elasticity, and catalytic performance showed to depend on the mesoscopic architecture, i.e., pore size, wall size, and pore connectivity. Especially a pore size in the large mesopore regime (around 40 – 50 nm) seems particularly beneficial because of the compromise between a still high surface area and a facile inter-pore diffusion. Since there are only a few block copolymer candidates to achieve such large pores due to a generally low solubility of large polymers, we investigated the underestimated copolymer class of poly(ethylene oxide)-block-poly(hexyl acrylate) (PEO-b-PHA) – initially reported by Lokupitiya et al. – as soft templates. Indeed, mesopores of 40 nm in size were obtained in both SiO2 and ZrO2 powders and thin films. Using physisorption, SAXS, STEM-based tomography, and ToF-SIMS, a detailed model of the pore structure was derived comprising 42 nm spherical pores, which are connected to 3 – 4 adjacent pores via 15 nm pore windows. In addition, further pore connection occurs over 1 – 2 nm wide pore channels penetrating the 17 nm thick pore wall. While in case of SiO2, the latter small pores lead to a poor overall pore connectivity, the pore system in ZrO2 is readily accessible likely due to a pore widening upon crystallization. This renders PEO-b-PHA-derived crystalline metal oxides a promising blueprint for further studies on catalytically relevant materials. Especially, a pore size tuning between 20 and 50 nm by adjusting the PHA block length shown in this work enables systematic studies on porosity-property relationships for e.g., optimizing stability and activity of electrocatalysts.
This picture was submitted by Lysander Wagner (group of Prof. Dr. Bernd Smarsly).
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