Tunable sol-gel synthesis of TiO2 aerogels with adjustable surface area and open porosity applied in photocatalytic reactions

Abstract

TiO2 aerogels are nanostructured materials which offer many advantages regarding the microstructural and semiconducting properties favored for photocatalytic applications. Active reaction sites on the high surface area of TiO2 aerogels are much better accessible through the high porosity of the material. This characteristic favors the adsorption and diffusion of reactants. Besides, the interconnected network of the aerogels, which offers long diffusion pathways for photogenerated electrons, the structural defects e.g. surface defects provide charge carrier traps for electrons leading to an improved charge separation and charge carrier storage capability. A novel acid-catalyzed sol-gel synthesis route for mesoporous TiO2 aerogels is presented. High surface areas and pore volumes can be obtained for the as-synthesized TiO2 aerogels based on alkoxide precursor hydrolysis and condensation with subsequent supercritical drying. X-ray diffraction and TEM analysis reveal the semi-crystalline character of the as-synthesized TiO2 aerogel besides the typical mesoporous structure and high surface area with narrow pore size distribution. Varying calcination temperatures of the TiO2 aerogels lead to a decrease of porosity and surface area but an increase of crystallinity. Photocatalytic experiments show the ability of the aerogels to store photogenerated electrons over several hours. This can be related to the high surface area and presumably high amount of surface defects. XPS results of the as-synthesized aerogel reveal the presence of Ti3+ states which is associated with surface defects e.g oxygen vacancies serving as electron traps. The effect of electron storage decreases with temperature treatments of the as-synthesized TiO2 aerogel, whereas the hydrogen evolution rate increases. This can be traced back to an improved interconnection of the TiO2 nanocrystals and therefore improved charge separation. The presented TiO2 aerogel is a potential candidate also for other photocatalytic reactions e.g. nitrogen reduction to ammonia (A. Rose et al, ChemRxiv, DOI: 10.26434/chemrxiv-2021-vxkpq-v2, 2022).