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Electron traps control the mobility of electrons in TiO2

Excess electrons in TiO2 form polarons (P), electron clouds trapped in Ti sites, detected as bright spots in Scanning Tunneling Microscopy

A combined theoretical (University of Vienna, Computational Materials Physics) and experimental (TU Vienna, Surface Physics) study published in Physical Review Letters uncovers the different nature of excess electrons in the two technologically most relevant polymorphs of titanium dioxide, rutile and anatase.

When extra charge carriers are introduced into an oxide material, their mobility is strongly determined by the coupling with the ion vibrations. If the strength of the electron-ion interaction is weak, an electron retains its free-carrier character, but for strong coupling it can be trapped in specific lattice sites forming a polaron. This concept is a key for virtually all prop­erties and application of oxides (catalytic activity, oxide electronics, photo­electrochemical solar cells) and its understanding poses challenging issues to material scientists. Electron trapping is central for the physical properties of the model semiconducting oxide TiO2.

Two forms of TiO2 are used industrially, rutile and anatase. The anatase form exhibits a better performance in energy-related applications and in optoelectronics. Even after several decades of research, the understanding of the origin of the difference between the two polymorphs has remained elusive.

In a study published in Physical Review Letters a team of physicists from the University of Vienna (C. Franchini, K. Merzuk and G. Kresse) and the TU Vienna (M. Setvin, X. Hao, M. Schmid and U. Diebold) in collaboration with the University of California (A. Janotti and Chris G. Van de Walle) has disclosed the intrinsic differences of excess electrons in TiO2 elucidating the fundamental basis of a long-standing debate in materials research. This was made possible by the combined use of scanning tunneling microscopy and spectroscopy, density functional and many-body theory.

It was found that the electrons are easily trapped by the rutile lattice, while they remain delocalized in anatase and can only be trapped near oxygen vacancies or form shallow donor states bound to typical dopants like Niobium. This result explains why TiO2 anatase is the perfect anode material in electrochemical solar cells, while the same material in rutile form provides only poor efficiency. Or why mixed anatase-rutile powders are the best photocatalysts. From a more fundamental point of view, the degree of electron localization in oxides is an unresolved, and highly controversial, issue in computational materials science and this work brings important methodological advances in this field.

Publication:

Direct View at Excess Electrons in TiO2 Rutile and Anatase
M. Setvin, C. Franchini, X. Hao, M. Schmid, A. Janotti, M. Kaltak, C. G. Van de Walle, G. Kresse, and U. Diebold
Phys. Rev. Lett. 113, 086402 (2014)
DOI: 10.1103/PhysRevLett.113.086402

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