Towards a More Efficient Use of Solar Energy

April 14, 2011

KARLSRUHE, Germany, April 14, 2011 /PRNewswire/ — The exploitation and
utilization of new energy sources are considered to be among today’s major
challenges. Solar energy plays a central role, and its direct conversion into
chemical energy, for example hydrogen generation by water splitting, is one
of its interesting variants. Titanium 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
Institute for Functional Interfaces (IFG) have studied the basic mechanisms
of photochemistry by the example of titania and have presented new detailed

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 involved cannot yet be described in
a satisfactory way. Together with colleagues from the universities of St.
(Scotland) and Bochum and Helmholtz-Forschungszentrum Berlin,
scientists at KIT’s Institute for Functional Interfaces, headed by Professor

Christof Woll, have succeeded in gathering new findings on the fundamental
mechanisms of photochemistry on titanium dioxide (TiO2).

Titanium dioxide, or titania, is a photoactive material occurring in
nature in the rutile and anatase modifications, the latter of which being
characterized by a ten times higher photochemical activity. When the white
TiO2 powder, which is also used as a pigment in paints and sunscreens, is
exposed to light, electrons are excited and can, for example, split water
into its components oxygen and hydrogen. The hydrogen produced in that way is
a “clean” energy source: No climate-killing greenhouse gases are generated
but only water is produced during combustion. Titanium dioxide is also used
to manufacture self-cleaning surfaces from which unwanted films are removed
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 photochemical reactions on
titania surfaces and especially the reason for the much higher activity of
anatase could not be explained. The powder particles used in photoreactors
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 substrates, 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. According to Professor Christof Woll, 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 prevents “internal short circuits”.
Knowledge of this feature will allow the researchers to further optimize
shape, size, and doping of anatase particles used inside photoreactors. The
objective is to develop photoactive materials with higher efficiencies and
longer lifetimes: “The results obtained by Professor Woll and his co-workers
are of great importance regarding the generation of electrical and chemical
energy from sunlight, and especially regarding the optimization of
photoreactors,” says Professor Olaf Deutschmann, spokesman of the Helmholtz
Research Training Group on “Energy-related Catalysis”.

The results obtained by the researchers have been published in Physical
Review Letters. The online version of the paper is available on

Mingchun Xu, Youkun Gao, Elias Martinez Moreno, Marinus Kunst, Martin
, Yuemin Wang, Hicham Idriss, Christof Woll, Phys. Rev. Lett. 106,
138302 (2011)

Karlsruhe Institute of Technology (KIT) is a public corporation according
to the legislation of the state of Baden-Wuerttemberg. It fulfills the
mission of a university and the mission of a national research center of the
Helmholtz Association. KIT focuses on a knowledge triangle that links the
tasks of research, teaching, and innovation.

This press release is available on the internet at http://www.kit.edu .

The photo of printing quality may be downloaded under http://www.kit.edu
or requested by mail to presse@kit.edu or phone +49-721-608-47414.

    Further contact:

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SOURCE KIT Karlsruher Institut fuer Technologie

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