Porosity tuning in soft-templated mesoporous silica : the influence of block copolymer composition and concentration

Abstract

Macroscopic properties of mesoporous metal oxides depend on the mesopore architecture, i.e., the pore size, wall thickness, and pore connectivity. Consequently, rational preparation protocols and deep knowledge of the templating mechanism are required for systematic porosity–property studies and the intentional synthesis of an optimized pore morphology. In this work, we thus prepared a library of 17 poly­(ethylene oxide)-block-poly­(hexyl acrylate) (PEO-b-PHA) block copolymers of varying PEO and PHA block lengths to quantitatively deduce the effect of the individual block lengths on the mesopore size of the templated silica. The block length of the hydrophobic PHA block in the micelle core showed to enable a pore size tuning between 10 and 80 nm according to electron microscopy, physisorption, and small-angle X-ray scattering. In contrast, varying the PEO block length did not alter the pore size, but revealed that a sufficiently large PEO block is required to ensure ordered spherical mesopores. Electron tomography confirmed a spherical pore geometry and a systematic decrease in pore wall thickness upon increasing the template concentration (i.e., template-to-silica ratio) during soft templating. A statistical in-depth analysis by tomography demonstrated that this wall size decrease is accompanied by an improved pore connectivity (e.g., in terms of the coordination number of adjacent pores) and an increasing pore size. The pore size increase originates from a partial PEO collapse on the micelle core based on a pore volume analysis and occurs only above a certain threshold concentration of block copolymer. We demonstrated that this concentration can be elevated by applying soft templates featuring shorter PEO blocks, which extend the regime of wall size tuning under preservation of pore dimension and shape. Overall, these insights provide a guideline on how to tailor the pore size, wall thickness, and pore connectivity of mesoporous metal oxides and enable systematic studies concerning the optimum porosity, e.g., for electrocatalytic applications to maximize stability and activity.