Fermi Level Positions and Induced Band Bending at Single Crystalline Anatase (101) and (001) Surfaces: Origin of the Enhanced Photocatalytic Activity of Facet Engineered Crystals

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 difference leads to a potential difference (band bending) at the interface between the (101) and (001) facets, resulting in the migration of photogenerated electrons and holes to different directions namely to the (101) and (001) facets, respectively. Photoelectron spectroscopy measurements clearly indicate for the first time that differences in the surface electronic structure and related potential of different facets explain a vectorial electron-hole pair separation to different reaction sites providing design criteria for enhanced photocatalysis.