R. Van De Krol et al., Electrical and optical properties of TiO2 in accumulation and of lithium titanate Li0.5TiO2, J APPL PHYS, 90(5), 2001, pp. 2235-2242
Changes in the optical absorption and electrical conductivity of dense and
mesoporous anatase TiO2 films were measured in situ as a function of electr
ode potential during electrochemical lithium intercalation. A special two-e
lectrode geometry was used for the conductivity measurements, in which the
contacts were separated by a small gap bridged by the TiO2. When electrons
are injected, an accumulation layer is formed and the conductivity increase
s several orders of magnitude. A monotonic increase of the optical absorpti
on with wavelength confirms the presence of (partially) delocalized electro
ns. Insertion of lithium ions results in the formation of the Li0.5TiO2 pha
se and a decrease of the overall conductance. The specific conductivity of
the Li0.5TiO2 phase is (9.1 +/-0.2) S/cm, significantly lower than that of
Li-doped anatase TiO2. This is corroborated by the absorption spectrum of L
i0.5TiO2, which shows two pronounced peaks around 440 and 725 nm and no cha
racteristic free-electron features. At potentials below 1.7 V vs Li additio
nal lithium could be inserted into Li0.5TiO2, and the conductance was found
to increase again. The results are discussed in terms of possible changes
in the electronic structure that are induced by the insertion of lithium io
ns. (C) 2001 American Institute of Physics.