The crystal phase, morphology and facet significantly influence the catalytic and photocat- alytic activity of TiO2. In view of optimizing the performance of catalysts, extensive efforts have been devoted to designing new sophisticate TiO2 structures with desired facet exposure, necessitating the understanding of chemical properties of individual surface. In this work, we have examined the photooxidation of methanol on TiO 2 （011）- （ 2 × 1 ） and TiO 2 （110） - （1 ×1） by two-photon photoemission spectroscopy （2PPE）. An excited state at 2.5 eV above the Fermi level （EF） on methanol covered （011） and （110） interface has been detected. The excited state is an indicator of reduction of TiO2 interface. Irradiation dependence of the excited resonance signal during the photochemistry of methanol on TiO2（011）-（2×1） and TiO2（110）-（1× 1） is ascribed to the interface reduction by producing surface hydroxyls. The reaction rate of photooxidation of methanol on TiO2（110）-（1× 1） is about 11.4 times faster than that on TiO2（011）-（2×1）, which is tentatively explained by the difference in the surface atomic configuration. This work not only provides a detailed characterization of the electronic structure of methanol/TiO2 interface by 2PPE, but also shows the importance of the surface structure in the photoreactivity on TiO2.
The electronic structure of methanol/TiO2（ll0） interface has been studied by photoemis- sion spectroscopy. The pronounced resonance which appears at 5.5 eV above the Fermi level in two-photon photoemission spectroscopy （2PPE） is associated with the photocatalyzed dissociation of methanol at fivefold coordinated Ti sites （Ti5c） on TiO2 （110） surface [Chem- ical Science 1, 575 （2010）]. To check whether this resonance signal arises from initial or intermediate states, photon energy dependent 2PPE and comparison between one-photon photoemission spectroscopy and 2PPE have been performed. Both results consistently sug- gest the resonance signal originates from the initially unoccupied intermediate states, i.e., excited states. Dispersion measurements suggest the excited state is localized. Time-resolved studies show the lifetime of the excited state is 24 fs. This work presents comprehensive char- acterization of the excited states on methanol/TiO2（110） interface, and provides elaborate experimental data for the development of theoretical methods in reproducing the excited states on TiO2 surfaces and interfaces.
