No short circuit in titanium oxide

Clarified fundamental mechanisms of photocatalysis of titanium oxide

Titanium dioxide can convert light into chemical energy. The modification anatase (b) of this oxide is much more efficient than rutile (a). © Mingchun Xu, Ruhr-University Bochum
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The molecule titanoxide is a photo catalyst: excited by sunlight, it can split water and thus generate chemical energy. How this happened was previously unknown, and now a research team has for the first time gained insights into the fundamental mechanisms of photochemistry of titanium oxide. Among other things, the new findings could be used to optimize photoreactors that generate hydrogen with sunlight.

The energy of sunlight can be used not only in solar thermal or photovoltaic, but also for the direct conversion of solar energy into chemical energy, for example by splitting water. The most efficient but poorly understood process is titanium oxide-based photocatalysis. Titanium dioxide is a photoactive material and occurs naturally in the two modifications rutile and anatase, with the anatase form having a tenfold higher photochemical activity.

Excited electrons split water

If light falls on this white powder, which is also used as a pigment in painting and as a sunscreen, electrons are put into excited states and can then, for example, split water into its components oxygen and hydrogen. The hydrogen produced in this way is a "clean" source of energy, because when it burns, it does not produce any climate-damaging greenhouse gas, but only water. The physical mechanisms of these photochemical reactions on titanium dioxide surfaces and in particular the reason for the much higher activity of anatase have not yet been elucidated, since the powder particles used for this are tiny with just a few nanometers.

Investigations on the single crystal

Such small particles are not suitable for investigation with powerful methods of surface analysis. Under the direction of Professor Christof Wöll, scientists from the KIT Institute for Functional Interfaces (IFG), in collaboration with colleagues from the Universities of St. Andrews (Scotland) and Bochum as well as the Helmholtz Research Center Berlin, have succeeded in gaining new insights into fundamental mechanisms photochemistry of titanium dioxide (TiO2) to win.

For their measurements, the researchers used millimeter-sized single crystals used. Precise measurements of the photochemistry on the surface of titanium dioxide could then be carried out on such substrates for the first time with the aid of a novel infrared spectrometer. In addition, scientists used a laser-based technique to determine the lifetime of light-generated electronic excitations inside TiO2 crystals. display

Anatase prevents internal short circuit

Accurate information about these processes is of great importance, as Christof W ll, head of the IFG, explains: A short life means that the excited electrons immediately return to their original state fall. It creates a kind of internal short circuit. With a long lifetime, the electrons remain in the excited state for long enough to reach the surface of the crystal, where they initiate chemical processes.

Anatase is particularly well suited for this because the electronic structure of this material has a special feature that prevents this "internal short circuit". The knowledge of this cause will allow researchers to further optimize the shape, size and doping of the anatase particles used in the photoreactors. The goal is to develop photoactive materials with higher efficiencies and longer lifetimes.

DieFor the generation of electrical and chemical energy from sunlight, the results of W ll and co-workers are of great importance, especially with regard to the optimization of photoreactors, Professor says Professor Olaf Deutschmann, spokesman of the Helmholtz Research Training Group "Energy-related Catalysis".

(Karlsruhe Institute of Technology, 15.04.2011 - NPO)