TiO₂(110) Charge Donation to an Extended π-Conjugated Molecule

Abstract

The surface reduction of rutile TiO2(110) generates a state in the band gap whose excess electrons are spread among multiple sites, making the surface conductive and reactive. The charge extraction, hence the surface catalytic properties, depends critically on the spatial extent of the charge redistribution, which has been hitherto probed by small molecules that recombine at oxygen vacancy (Ovac) sites. We demonstrate by valence band resonant photoemission (RESPES) a very general charge extraction mechanism from a reduced TiO2(110) surface to an extended electron-acceptor organic molecule. Perylene-tetra-carboxylic-diimide (PTCDI) is not trapped at Ovac sites and forms a closely packed, planar layer on TiO2(110). In this configuration, the perylene core spills out the substrate excess electrons, filling the lowest unoccupied molecular orbital (LUMO). The charge transfer from the reduced surface to an extended π-conjugated system demonstrates the universality of the injection/extraction mechanism, opening new perspectives for the coupling of reducible oxides to organic semiconductors and supported catalysts.

X-ray standing waves reveal lack of OH termination at hydroxylated ZnO(0001) surfaces

“…From ultraviolet photoelectron spectroscopy (UPS), additional occupied density of states in the gap between highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO) in the interface regime is apparent [27,29,40]. These observations are equivalent to those reported for PTCDI/TiO 2 , for which a deprotonation reaction of PTCDI at the oxide surface was suggested as possible origin [41]. However, the same observations were also reported for C 4 -PTCDI on ZnO [27], for which the N atoms are bound to butyl groups and a deprotonation reaction is not possible.…”

Scanning probe microscopy studies on the adsorption of selected molecular dyes on titania

“…The difference in the molecular structure between PTCDA and PTCDI seems to be small, i.e., an oxygen atom linking carbon atoms in an anhydride group is exchanged by pyrrolic (N–H) nitrogen. However, the local molecular orientation of PTCDI molecules on the TiO 2 (110) surface only partially resembles that observed for PTCDA molecules [28,35]. The long molecular axis in case of both PTCDI and PTCDA is parallel to the [001] direction.…”

“…It is worth to mention quite recent results reported for another perylene-derivative, namely perylene di-imide (PTCDI) [34–35]. The difference in the molecular structure between PTCDA and PTCDI seems to be small, i.e., an oxygen atom linking carbon atoms in an anhydride group is exchanged by pyrrolic (N–H) nitrogen.…”

“…Such delocalized charges may be extracted from the surface and lead to reduction of molecular adsorbates. Lanzilotto et al [35] have shown that the interaction range of the excess electrons is limited to a single unit cell, i.e., ca. 1 nm 2 , and, hence, reduced PTCDI is identified as single molecules within the (1 × 5) islands.…”

“…As Lanzilotto et al [35] have shown, the interaction range for these electrons is limited to a single unit cell. Consequently, when considering a molecular layer only single molecules are influenced by the subsurface delocalized charges.…”