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Solar Cells // More Efficient Use of Solar Energy

T

itanium oxide-based photocatalysis is the presently most efficient, yet little understood

conversion process. In cooperation with colleagues from Germany and abroad, scientists of the KIT In- stitute for Functional Interfaces (IFG) have studied the basic mechanisms of photochemistry by the example of titania and have presented new detailed findings.

Even though hydrogen production from water and sunlight by means of oxide powders has been studied extensively for several decades, the basic physical and chemical mechanisms of the processes invol- ved cannot yet be described in a satisfactory way. Together with colleagues from the universities of St. Andrews (Scotland) and Bochum and Helmholtz- Forschungszentrum Berlin, scientists at KIT’s Insti- tute for Functional Interfaces, headed by Professor Christof Wöll, have succeeded in gathering new findings on the fundamental mechanisms of photo- chemistry on titanium dioxide (TiO2

).

Titanium dioxide, or titania, is a photoactive material occurring in nature in the rutile and anatase modi- fications, the latter of which being characterized by a ten times higher photochemical activity. Titanium dioxide is also used to manufacture self-cleaning surfaces from which unwanted films are remo- ved through photochemical processes triggered by incident sunlight. In hospitals, this effect is used for

sterilizing specially coated instruments by means of UV irradiation.

So far, the physical mechanisms of these photoche- mical reactions on titania surfaces and especially the reason for the much higher activity of anatase could not be explained. The powder particles used in pho- toreactors are as tiny as a few nanometers only and are thus too small for use in studies by means of the powerful methods of surface analysis.

By using instead mm-sized single-crystal sub- strates, the researchers were for the first time able to precisely study photochemical processes on the surface of titanium dioxide by means of a novel infrared spectrometer.

Using a laser-based technique, the scientists, in addition, determined the lifetime of light-induced electronic excitations inside the TiO2

crystals. Ac-

cording to Professor Christof Wöll, Head of the IFG, exact information about these processes is of great importance: “A short lifetime means that the excited electrons fall back again at once: We witness some kind of an internal short circuit. In the case of a long lifetime, the electrons remain in the excited state long enough to be able to reach the surface of the crystal and to induce chemical processes.” Anatase is particularly well suited for the latter purpose because it is provided with a special electronic structure that

11-04 :: April/May 2011