Polycrystalline anatase thin films, (001)-and (101)-oriented anatase TiO 2 single crystals and (001)-and (110)-oriented rutile TiO 2 single crystals with various surface treatments were studied by photoelectron spectroscopy to obtain their surface potentials. Regardless of orientations and polymorph, a huge variation of the Fermi level and work function was achieved by varying the surface condition. The most strongly oxidized surfaces are obtained after oxygen plasma treatment with a Fermi level ∼2.6 eV above the valence band maximum and ionization potentials of up to 9.5 eV (work function 7.9 eV). All other treated anatase surfaces exhibit an ionization potential independent of surface condition of 7.96 ± 0.15 eV. The Fermi level positions and the work functions vary by up to 1 eV. The ionization potential of rutile is ∼0.56 eV lower than that of anatase in good agreement with recent band alignment studies.

Pt and PbO 2 on the specific orientations of rutile and anatase via photodeposition, indicating that the facets help in the separation of photoinduced electrons and holes. It was found that electrons tend to be transferred to the (101) facets, whereas the holes are driven to the (001) surfaces. This result suggests that anatase (101) surface provides the effective reduction site, whereas anatase (001) works as the oxidation site. Tachikawa et al. [5] investigated facet dependant photocatalysis on anatase with single-molecule fluorescence imaging and kinetic analysis by using redox-responsive fluorogenic dyes. On the single crystal of anatase coexposed with the (101) and (001) facets, the fluorogenic dyes are preferentially reduced on the (101) facet rather than the (001) facet. This finding confirms that photogenerated electrons preferentially migrate to and are trapped at the (101) facet. Such a charge carrier separation was observed for different metal oxides such as Cu 2 O, WO 3 , and BiVO 4 . [3,6] Furthermore, based on the charge separation between different facets in the crystal, Li et al. [7] demonstrated a drastic enhancement of photocatalytic activities by selectively depositing reduction and oxidation cocatalysts onto the reductive and oxidative facets of BiVO 4 crystals. In summary, there is a strong need of deeper understanding of the mechanism of charge separation between different crystal facets.Surface properties of the TiO 2 anatase have been studied by a number of experimental and theoretical investigations without providing a clear reason for different photocatalytic efficiencies. [8] However, the charge separation and trapping are conventionally explained by the different energy levels of different facets due to the surface atomic arrangement and coordination. [9] Recently, a first-principles calculation predicted that the Fermi level of the (001) facet is located at a lower energy level than that of the (101) facet. [9a] Thus, a so-called surface heterojunction would be formed between the (101) and (001) facets due to the original difference of their surface Fermi levels in a crystal exposed with both facets. As a result, photogenerated electrons and holes could preferentially migrate to the (101) and (001) facets, thereby exhibiting different photocatalytic activities on these facets. However, the Fermi level shown in the density of states is located near valence band maximum for the (101) and enters even into valence band for the (001) surface meaning that the (101) surface is a p-type semiconductor and Single crystalline anatase is used to prepare well defined (001) and (101) surfaces in ultrahigh vacuum (UHV) in different states: sputtered, annealed, stoichiometric, and oxidized. The electronic properties of the well-defined surfaces are investigated by X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy after UHV transfer. The Fermi level of (001) facets for all applied surface conditions is lower than that of the (101) facets by 150-450 meV. The energy differen...