Photocatalytic Activity and Electron Storage Capability of TiO2 Aerogels with an Adjustable Surface Area

Abstract

TiO2 aerogels are suitable candidates for photocatalytic applications due to their high surface area with presumably high amount of reactive sites and improved charge separation. Furthermore, TiO2 in general is known for the trapping of electrons close to the surface with the formation of Ti3+ states, which shows a characteristic broad absorption at a maximum of 650 nm and dark blue coloration. These stored electrons can be used for different reduction reactions, like the reaction with different metal ions to metal nanoparticles.A very promising dark reduction reaction for these stored electrons is the nitrogen reduction reaction, which was reported the first time in 2011 for TiO2 colloids. Herein, we present a detailed investigation of electron storage capability and photocatalytic activity of varying mesoporous TiO2 aerogels. These aerogels were prepared via a novel modified acid catalyzed sol-gel synthesis with subsequent supercritical drying and different calcination treatments. Absorption measurements revealed an increased storage of photogenerated electrons upon sunlight irradiation) in water-methanol mixtures with decreasing calcination temperature and increasing surface area, which goes in line with a decreased hydrogen evolution rate. The electron storage capability further depends on the concentration of the hole scavenger. Reduction reaction of photogenerated electrons in the aerogels in the dark are shown on the one hand by the reduction of Pt ions in hydrogen evolution experiments, and on the other hand by nitrogen reduction to ammonia verified by salicylate test.