Antenna mechanism and deaggregation concept: novel mechanistic principles for photocatalysis

TiO2 photocatalysis has attracted considerable scientific and practical interest during the last three decades. One major current direction in this research area is the design and preparation of novel TiO2-based photocatalysts possessing high photocatalytic activity to satisfy the requirements for practical applications. For the development of successful strategies to achieve this goal it is, however, of utmost importance to improve the mechanistic understanding of the basic principles of photocatalysis. Therefore, various nanosized TiO2 photocatalysts including pure TiO2, Fe(III)-doped TiO2 and platinized TiO2 have been synthesized in our laboratories and their photocatalytic activity has been studied in detail. Several unexpected results have been obtained during these investigations and two new mechanistic models have consequently been developed for their explanation. On the one hand, it was observed that Fe(III)-doped TiO2 nanoparticles form extended networks in aqueous suspensions with the particles being attached in a way that their atomic planes are aligned parallel to one another. Through the so-called Antenna Mechanism the resulting energetic coupling throughout a long chain of titania nanoparticles will enable an energy and/or exciton transfer from the particle where the initial photon absorption took place to the particle where the electron transfer process finally occurs. On the other hand, the so-called deaggregation concept was introduced to explain a 50% increase of the quantum yield when methanol was photocatalytically oxidized in aqueous suspensions of platinized TiO2 nanoparticles employing pulsed laser excitation instead of continuous illumination. Within each individual laser pulse, the ensemble of nanoparticles absorbs sufficient energy to initiate the deaggregation of the particle network held together by hydrogen bonds thus opening up additional surface area for methanol adsorption and oxidation.