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May 22nd, 2014
 
EPFL scientists have shown how electrons move and get trapped in titanium dioxide
 
iO2 is used in Grätzel cells, self-cleaning windows and water-purifying technologies
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Titanium dioxide (TiO2) is the material most used in photovoltaic dye-sensitized solar cells, as well as in photocatalytic applications, such self-cleaning glass. In both technologies, the underlying principle is the creation of electrical charges, electrons (negative) and holes (positive). However, the fate of these charges after their creation remains a fundamental question. Using a unique method, EPFL researchers have for the first time determined the nature of the electron traps in TiO2, which can significantly optimize its use in light-based technologies. Their work is published in Angewandte Chemie.

Converting light into electrical or chemical energy is one of the major goals of our time, with enormous implications on the future of mankind. Applications include photovoltaic devices like dye-sensitized solar cells (DSSC; Grätzel cells) and photocatalytic devices like self-cleaning windows and water-purification systems. Most devices are made of a crystalline form of TiO2 called anatase, which is often sensitized with a light-absorbing dye molecule that injects electrons into the TiO2 substrate.

In photovoltaic devices, the electrons are the main actors, while for photocatalysis, they reduce impurities that attach to a surface or the electron holes, and oxidize them by, respectively, adding or removing electrons from these impurities. Thus, the efficiency of photovoltaic and photocatalytic devices depends not only on the transport of charges in the TiO2 lattice, but also on the fact that they are trapped in the right place. In the case of electrons, the nature (location, geometry, etc.) of the traps and their lifetime, particularly at room temperature, has remained a mystery.

The group of Majed Chergui at EPFL has used time-resolved X-ray absorption spectroscopy (XAS) to study how electrons are trapped in the anatase (ordered) and amorphous (disordered) forms of TiO2 at room temperature. XAS is a powerful, element-specific technique for determining the electronic structure of atoms and the geometric structure around them.

The results show that in both bare and dye-sensitized anatase, electrons get trapped in the defect-rich surface shell of the TiO2 substrate; deep inside the shell in the case of bare anatase and on the outer surface in the case of dye-sensitized nanoparticles. For bare anatase nanoparticles, the electrons can travel through the regular lattice towards the surface, where they become trapped as soon as they encounter defects, which is the case deep inside the surface shell. The travel time of the electron is well below the time resolution of the experiment (100 ps). For dye-sensitized nanoparticles, the electrons are immediately trapped at the surface, because defects are dominant there.
 

 
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