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

Abstract

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...

Band positions of anatase (001) and (101) surfaces in contact with water from density functional theory

“…This is similar to the experimental photoelectron spectroscopy results of Kashiwaya et al 18 : Their study on differently prepared single-crystals of anatase with X-ray and ultraviolet photoemission in vacuum find occupied surface states slightly above the bulk valence band maximum for the (001) surface that can be -apart from a shallow valence band surface state -largely suppressed by dedicated sputter-annealing routines followed by reoxidation. Their (001) surface without reoxidation still comprises a deep band gap state very close to the middle of the band gap.…”

“…12 Experimentally, photoelectron spectroscopy at differently treated (101) and (001) surfaces of single-crystals evidenced a band offset favouring electron migration to the (101) facets, whose magnitude depends on prior sample treatment. 18,19 Such a driving force could help in improving the catalytic activity of TiO 2 nanoparticles or improve the charge-transport over hetero-interfaces for TiO 2 -based protection layers. A key question, to be addressed in this study, is to what extent the contact with water modifies this surface electronic structure behaviour.…”

Intrinsic intermediate gap states of TiO2 materials and their roles in charge carrier kinetics

“…Studying charge carrier kinetics and their effects on photocatalysis is highly interesting to us; this is complicated when TiO 2 materials are in forms of nanostructures and contain significant defects [39][40][41]. In principle, the charge carrier kinetics are mainly affected by the electronic structures of TiO 2 materials, which are also shown to be related to the crystalline phases [42], exposed facets [43], interparticle boundaries [44][45][46], interfaces exposure to adsorbates [47], energy band bending [48], and others. [49,50].…”

On the importance of catalysis in photocatalysis: Triggering of photocatalysis at well-defined anatase TiO2 crystals through facet-specific deposition of oxygen reduction cocatalyst

“…[16][17][18][19] Secondly, anatase TiO2 crystals with coexposed {101} and {001} were suggested to exhibit anisotropic transport of photogenerated charges, electrons to {101} and holes to {001} facets, which should lead to enhanced charge separation. [20][21][22][23][24][25][26][27] As an explanation for this anisotropy, both bulk properties (e.g., anisotropic effective mass of electrons and holes) 26 and formation of a "surface heterojunction" at the interface of {101} and {001} 25,27,28 have been proposed.…”