Rutile TiO2 is a prototypical light-harvesting semiconductor showing promising potential in the application of photocatalysis. Nevertheless, it has proven difficult to establish the detailed processes involved in photocatalysis, partly because the excited states involved are difficult to study. Here, we present evidence for the existence of hydroxyl-induced states in the conduction band region that may also play a role in photocatalysis. In a two-photon photoemission (2PPE) experiment on rutile TiO2(110), an electron in the well-known defect states about 0.8 eV below EF is excited into an intermediate state by absorbing a photon, from which it is further excited above the vacuum level by a second photon. By varying the photon energy from about 3 to 4 eV, we observe a pronounced resonant enhancement of the 2PPE intensity when the photon energy is about 3.5 eV. It indicates an intermediate state at about 2.7 eV above EF. Further investigations show that this state is induced by the hydroxyls on the TiO2 surface with a lifetime shorter than 15 fs. The intensity of the resonance can be greatly enhanced by the adsorption of molecular water at 100 K. The energy of the hydroxyl-induced state is found to be 2 eV lower and less-influenced by water adsorption than that expected for a hydroxylated TiO2(110) surface.

Non-band-gap photoexcitation of hydroxylated TiO2

251st National Meeting (2016)

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