Photocatalytic Activity and Electron Storage Capability of TiO2 Aerogels with Adjustable Surface Area
Authors: Hofmann Anja , Marschall Roland , Voepel Pascal , Rose Alexandra , Milow Barbara 
Published: February 2024

Nanostructuring of semiconductor photocatalysts is one important strategy to improve photocatalytic activities. An interesting nanostructuring strategy is the generation of mesoporosity, i.e. in aerogels. Aerogels are 3D nanostructures of interconnected porous networks with enormous porosity, ultra-low density, and high surface areas, which are higher than surface areas of particle systems.[1,2] The high surface area and presumably high amount of reactive sites and a good contact of the active material to the electrolyte, as well as short diffusion pathways for the minority charge carriers and an improved charge carrier separation are advantageous for photocatalytic applications.[3-5]
TiO2 aerogels offer a higher density of photoexcited electrons compared to TiO2 nanoparticles.[5] Electrons in TiO2 are trapped close to the surface of the material and Ti3+ states are formed, which show a characteristic dark blue coloration, i.e. a broad absorption at a maximum of 650 nm.[6-8] Stored photoexcited electrons in TiO2 can be used for different dark reduction reactions.[7-10] A very promising reduction reaction using such stored photoexcited electrons is the nitrogen reduction reaction for the production of ammonia, which was first reported by Bahnemann et al. in 2011 for stored electrons in TiO2.[8] This reaction can pave the way to an energy efficient and decentralized production of ammonia, which is commonly produced via the extremely energy intensive Haber-Bosch process, which causes 2 % of the annual global energy consumption and 1.6 % of the global CO2 emissions.[11]
Herein, we present the photocatalytic hydrogen evolution activity and electron storage capability of mesoporous TiO2 aerogels which were prepared via a novel acid-catalyzed sol-gel synthesis with subsequent supercritical drying.[12] The as-synthesized aerogel provided a surface area of 600 m2 g-1 which could be tailored by different heat treatments. The sacrificial hydrogen evolution activity of the aerogels decreased with increasing surface area and decreasing crystallinity. At the same time, the addition of an aqueous H2PtCl6 solution to the bluish dispersions in the dark resulted in the evolution of hydrogen without light irradiation, due to the reduction of Pt4+ to Pt0 and the formation of a Schottky contact. The intensity of that dark hydrogen evolution peak increased with increasing surface area. This reduction reaction could be used to quantify the amount of electrons stored in the aerogels. Furthermore, it could be shown that the electron storage capability depends on the hole acceptor concentration. The ability to reduce nitrogen to ammonia in the dark with the stored photoexcited electrons in such an aerogel will be presented. These results pave the way for future research on the optimization of aerogel properties depending on their future application together with optimization of photocatalytic nitrogen reduction and other reduction reactions with such aerogels.
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