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

Abstract

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 . ■ INTRODUCTIONWide bandgap oxide semiconductor TiO 2 is still considered one of the best materials for photocatalysis and solar energy conversion, 1 while the anatase form is usually considered to be more active in photocatalytic and photovoltaic applications. The large bandgap of TiO 2 ensures the photogenerated electrons within the conduction band (CB) have a strong reducing ability and the holes in the valence band have a strong oxidizing ability. 2 Either in photocatalysis or in the photovoltaic process, nanophase TiO 2 has been used for its highly effective surface area as well as its large number of surface binding sites. However, the number of surface and interstitial defects known as trap states also increases substantially in the nanoparticles versus the number in the single crystals. These trap states with their energy levels lying in the bandgap act as carrier traps in competition with the fast carrier recombination in the bulk during photoexcitation, which enhances the photoactivity of the nanoparticles. On the other hand, the deep trap states reduce the photocatalytic activities when their chemical potentials are considered. 3 Because of its wide bandgap, TiO 2 absorbs light only in the UV range, which accounts for only 3−5% of the total sunlight; 4 this leads to a low light conversion efficiency in the solar spectral region. Therefore, extending the absorption of TiO 2 to the visible range would be an effective means of increasing its overall efficiency. One way to increase solar energy absorption efficiency is to narrow the bandgap by anion doping such as nitrogen doping. 5 At present, nitrogen-doped TiO 2 exhibits the strongest response to solar radiation, 6 but its absorption in the visible and infrared region remains insufficient.In contrast to the cation-doped TiO 2 , the surface and interstitial defects usually known in a form of Ti 3+ such as selfdoping 7 act as color centers; these color centers in principle would not narrow the bandgap but would provide a cha...

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