Determination of Midgap State Energy Levels of an Anatase TiO₂ Nanocrystal Film by Nanosecond Transient Infrared Absorption – Excitation Energy Scanning Spectra

Midgap levels for wide gap TiO 2 have become increasingly important because they can be used to capture solar light efficiently for photocatalysis as demonstrated by black TiO 2 in a recent paper [Chen, X., et al. Science 2011, 331 (6018), 746–750]. However, a method for systematically characterizing the midgap state energy levels is still lacking. We proposed an optical method, i.e., transient infrared (IR) absorption – excitation energy scanning spectrum, by recording nanosecond time-resolved transient IR absorption from the excited electrons either in the conduction band or at the excited localized states below the conduction in combination with midgap excitation energy scanning. We demonstrate that both the electron trap states beneath the Fermi level and those excited localized states below the conduction band as well as the Fermi level of TiO 2 nanoparticles can unambiguously be determined by this method, which has great potential for characterizing the midgap trap states of various semiconductor nanomaterials other than TiO 2 .