Electronic structure of lithium-doped anatase TiO2 prepared in ultrahigh vacuum

Insertion of lithium in anatase TiO2, giving LixTiO2, is performed under ultrahigh vacuum (UHV) conditions and studied using synchrotron radiation based electron spectroscopy. Core level photoemission spectra are directly compared to results obtained after electrochemical insertion, illustrating the usefulness of the UHV approach. The growth of a state of mainly Ti 3d character in the band gap is monitored and the amount of charge transferred from Li to the band gap state is quantified. The result that the Ti 3d level is occupied by 0.85±0.10 electronic charge is in good agreement with theoretical predictions. Binding energy shifts of the core levels suggest that the population of the Ti 3d states does not follow a simple rigid band behavior. It is concluded that the formation of the Li-poor phase (x<2%) is associated with pinning of the Fermi level to the bottom of the conduction band. The Li-poor phase can therefore be envisaged as related to defects. Changes in the valence photoemission spectrum and O 1s x-ray absorption spectrum are interpreted in terms of a decreased O 2pTi 3d interaction upon Li insertion. Shifts in the sample work function are finally found to agree reasonably well with the measured cell voltage for electrochemical Li insertion into a nanoporous anatase film.