Surface Investigations on Single Crystal Anatase TiO 2
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R. HENGERER*, L. KAVAN**, B. BOLLIGER***, M. ERBUDAK***, and M. GR TZEL* *Laboratory of Photonics and Interfaces, Swiss Federal Institute of Technology, CH-1015 Lausanne. **J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, CZ18223 Prague. ***Laboratory of solid state physics, Swiss Federal Institute of Technology, CH8093 Z rich.
ABSTRACT Utilizing a chemical transport reaction, we succeeded in growing large and clean anatase TiO 2 single crystals whose surfaces could be characterized by standard physical and electro-chemical techniques. The examination of the structure of the clean (101) and (001) faces by low energy electron diffraction (LEED) and secondary electron imaging (SEI) showed that these surfaces are bulk terminated and thermodynamically stable. Impedance spectroscopy in aqueous solution revealed a slight difference in the flatband potential between the (101) and the (001) faces. This shift is also manifested in a different photocurrent onset potential and can be rationalized by a different water adsorption on the two surface structures. Voltammetry in aprotic solutions showed a different lithium insertion behavior for the two surfaces. This is explained by a different structural transparency of the anatase lattice in the two directions. Both findings favor the (001) over the (101) surface. These orientational dependencies may have some important technological relevance for the mesoscopic TiO 2 films used in solar cells and lithium batteries.
INTRODUCTION TiO 2 is a wide-bandgap semiconductor that exists mainly in two polymorphic phases, anatase and rutile. So far, only surface properties of rutile are investigated systematically [1]. Due to the lack of suitably large and pure single crystals, investigations on specific surfaces are missing [2]. It is anatase, however, that plays a crucial role in a number of charge-separating devices, like dyesensitized solar cells [3] or rocking-chair lithium batteries [4]. These devices are based on highly porous films with very large surface areas that consist of nano-crystalline anatase mainly exhibiting the (101) and (001) faces. Although they show already a remarkable performance, fundamental understanding of surface processes, like dye adsorption or charge transfer, is still deficient. Moreover, charge-transfer processes should generally depend on the crystallographic orientation, so separate characterizations of different surfaces are desirable. Here we report the first successful imaging cf the surface lattice of TiO 2 single crystals in the anatase structure by secondary-electron imaging (SEI) and low-energy electron diffraction (LEED), respectively. On both faces, which are originally bulk-terminated, we have observed reversible structural transitions, induced by sputtering and subsequent annealing. In addition, electrochemical
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Mat. Res. Soc. Symp. Proc. Vol. 623 © 2000 Materials Research Society
and photoelectrochemical studies in aqueous media, as well as Li+ insertion experiments were performed. We point a
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