Optimization of Mass and Light Transport in Nanoparticle-Based Titania Aerogels.

Aerogels composed of preformed titania nanocrystals exhibit a large surface area, open porosity, and high crystallinity, making these materials appealing for applications in gas-phase photocatalysis. Recent studies on nanoparticle-based titania aerogels have mainly focused on optimizing their composition to improve photocatalytic performance. Little attention has been paid to modification at the microstructural level to control fundamental properties such as gas permeability and light transmittance, although these features are of fundamental importance, especially for photocatalysts of macroscopic size. In this study, we systematically control the porosity and transparency of titania gels and aerogels by adjusting the particle loading and nonsolvent fraction during the gelation step. Mass transport and light transport were assessed by gas permeability and light attenuation measurements, and the results were related to the microstructure determined by gas sorption analysis and scanning electron microscopy. Mass transport through the aerogel network was found to proceed primarily via Knudsen diffusion leading to relatively low permeabilities in the range of 10-10 m/s, despite very high porosities of 96-99%. While permeability was found to depend mainly on particle loading, the optical properties are predominantly affected by the amount of nonsolvent during gelation, allowing independent tuning of mass and light transport.