Clocking the Ultrafast Electron Cooling in Anatase Titanium Dioxide Nanoparticles

January 11, 2018

Eduardo Baldini, Majed Chergui and co-workers have shown that after electron delivery to the famous titanium dioxide material, the excess energy is lost to heat at extremely short time scales.

Transition metal oxides such as titanium dioxide (TiO₂) are at the center of the recent surge in research and development on solar-energy technologies. With solar cells and photocatalysis at the forefront, scientists are interested in understanding every step of the fate of charge carriers (electrons and holes) in transition metal oxides.

In solar energy conversion, sensitized solar cells have been the most explored systems. The cells consist of a dye molecule or a perovskite adsorbed onto the surface of a mesoporous TiO₂ film. The role of the sensitizer is to collect light and inject electrons into the film, which helps them migrate — this is, fundamentally, the generation of electrical current from light. The fate of the electron after delivery to the metal oxide is of crucial importance for optimization of the cells.

One way of investigating it is to use pure transition metal oxides excited above the band gap and observe the fate of the electron after this delivery scheme. This requires a high temporal resolution of femtoseconds (1 femtosecond is a millionth of a billionth of a second) but also a method that allows detecting and disentangling charge carriers, which were unavailable until recently.

The lab of Majed Chergui at EPFL, within the Lausanne Centre for Ultrafast Science, has now successfully probed the charge carrier-relaxation in TiO₂ nanoparticles after above band-gap photoexcitation. To achieve this, the researchers have pioneered a new technique, named ultrafast two-dimensional deep-ultraviolet spectroscopy. The technique allowed them to directly track electron cooling in the conduction band of the material on a time scale of less than 50 fs! This extremely short time scale is due to the strong coupling of the electrons to the lattice, which leads to the excess energy (and, therefore, the kinetic energy) of the electrons being rapidly dissipated into the form of heat.

These results call for new strategies of electron delivery by charge injection in sensitized systems. Indeed, large excess energies enhance the efficiency of charge injection, but the timescale involved in their dissipation prevents the carriers from being efficiently transported towards the electrodes. Taken from another perspective, the extremely fast dissipation of the carrier kinetic energy is preferable in photocatalysis and may be an important ingredient to explain why TiO₂ works as a very efficient photocatalyst.

“Revealing the subtle details of the charge carrier dynamics in this cheap and abundant material will also allow us to engineer new schemes for photonic applications,” says former EPFL physics PhD student Edoardo Baldini (now at MIT), who first-authored the study.

Reference: Baldini, E., T. Palmieri, E. Pomarico, G. Auböck and M. Chergui (2018). Clocking the Ultrafast Electron Cooling in Anatase Titanium Dioxide Nanoparticles. ACS Photonics. (10.1021/acsphotonics.7b00945) Baldini-2018.

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