Tailoring Mass and Light Transport in Nanoparticle-Based Aerogels for Gas-Phase Photocatalysis

Aerogels are among the most fascinating human-made materials due to their numerous extraordinary properties. These extremely lightweight materials not only have a very intriguing appearance but also are made up of a finely branched three-dimensional network that offers very high porosity and extraordinary surface area. Due to their structural characteristics, aerogels offer great potential for diverse applications such as detection and sensing, filtration, and catalysis. Aerogels have been fabricated for decades using conventional sol-gel chemistry, a process that, however, is limited in design flexibility. More recently, an alternative method for aerogel fabrication has emerged that uses pre-synthesized building blocks to create such highly porous three-dimensional assemblies.^[1] Since the properties of the nanoscale building blocks are typically preserved during the assembly process, the final aerogel can be precisely tailored to a specific application not only in terms of composition, but also in terms of crystallinity, size, shape, and surface chemistry of the underlying aerogel framework.<br/><br/>By using highly crystalline titanium dioxide nanoparticles, for example, transparent monolithic aerogels can be prepared that have a surface area of up to 500 m²/g and a porosity of up to 99 %. The photoactive backbone can be further decorated with selected co-catalyst nanoparticles to enhance the photocatalytic performance. The potential of particle-based aerogels has been demonstrated in numerous studies, centimeter-sized titania-metal aerogels, for example, have been explored in the photocatalytic conversion of gaseous carbon dioxide ^[2] and also show excellent activity in hydrogen production from organic feedstocks. ^[^3,4]<br/><br/>In the last decade, the focus has been centered on exploiting new building blocks and tuning their composition to improve light utilization or enhance catalytic activity. ^[5] In contrast, little attention has been paid to modification at the structural level. This is partly because the internal structure of aerogels is morphologically complex and difficult to characterize, but also because the parameters controlling particle assembly are not yet fully understood.<br/><br/>In this talk, titania-based aerogels will be used as an example to show how structural modifications of particle-based aerogels can be realized in order to tune two essential properties for gas-phase photocatalysis: gas permeability and light transmission. ^[6] Furthermore, it will be shown how measurements of gas permeability can not only serve as a tool to determine fundamental properties of mass transfer through such particle-based assemblies, but in conjunction with optical measurements, gas sorption analysis, and microscopy techniques can also provide valuable insights into the formation mechanism of this fascinating class of materials.<br/><br/>[1]. Matter F., Luna A. L., Niederberger M., Nano Today <b>2020,</b> <i>30</i>, 100827.<br/>[2]. Rechberger F., Niederberger M., Mater. Horiz. <b>2017,</b> <i>4</i> (6), 1115.<br/>[3]. Luna A. L., Matter F., Schreck M., Wohlwend J., Tervoort E., Colbeau-Justin C., Niederberger M., Appl. Catal. B <b>2020,</b> <i>267</i>.<br/>[4]. Kwon J., Choi K., Tervoort E., Niederberger M., J. Mater. Chem. A <b>2022,</b> <i>10</i> (35), 18383.<br/>[5]. Kwon J., Choi K., Schreck M., Liu T., Tervoort E., Niederberger M., ACS Appl. Mater. Interfaces <b>2021,</b> <i>13</i> (45), 53691.<br/>[6]. Matter F., Niederberger M., Adv. Sci. <b>2022,</b> <i>9</i> (13), 2105363.