Methanol has long been used as a hole scavenger in photocatalysis to improve charge separation. Although the prototypical methanol/TiO2(110) system shows salient adsorption-state-dependent photoactivity, the underlying reason still remains unclear. Through surface-sensitive ultraviolet photoelectron spectroscopy measurements, we found that methoxy anions and methanol strengthen and flatten the originally upward band bending of TiO2(110), respectively. Intensified upward band bending leads to charge separation in the depletion region and hole accumulation at the surface. Furthermore, density functional theory calculations show that the hole transfer at the CH3O–/TiO2 interface is thermodynamically allowed and that on CH3OH/TiO2 is unfavorable. The improved charge separation together with the allowed interfacial hole transfer is found to be responsible for the superior photoactivity of the methoxy anion over methanol on TiO2. These results reconcile the existing contradiction in the understanding of the charge transfer at the CH3O–/TiO2 interface based on the photoemission measured energy levels. Our work suggests that the redox potential level, rather than the vertical energy (for example, the HOMO of adsorbates) measured by spectroscopy, should be used to evaluate the possibility of the heterogeneous interfacial charge transfer.

The band gap state of TiO 2 , which is dominated by Ti 3+ 3d character, is of great relevance to light absorption, electron trapping, charge recombination, and conduction band structure. Despite the importance, the explanation of the excitation from this state is controversial. To this end, the electronic structures of TiO 2 (110) and TiO 2 ( 011)-(2 × 1) have been systematically measured with two-photon photoemission spectroscopy. The results reveal the anisotropic nature of the electronic structure in rutile TiO 2 at seemingly equivalent directions of [110] and [11̅ 0], the long axes of the TiO 6 blocking unit. Although the resonant energy of these two d−d transitions is identical, the energy levels are systematically shifted by 0.1 eV. We propose this anisotropy originates from the broken symmetry of the rutile TiO 2 crystals caused by the surface. The proposed asymmetry-caused electronic structure anisotropy could be generalized to other similar materials and may affect associated catalytic properties. This work provides an important benchmark for related calculations.

Band structure dictates optical and electronic properties of solids and eventually the efficiency of the semiconductor based solar conversion. Compared to numerous theoretical calculations, the experimentally measured band structure of rutile TiO2, a prototypical photocatalytic material, is rare. In this work, the valence band structure of rutile TiO2(110) is measured by angle-resolved photoelectron spectroscopy using polarized extreme ultraviolet light. The effective mass of the hole, which has never been measured before, is determined to be 4.66–6.87 m 0 (free electron mass) and anisotropic. The dependence of photoemission intensities on excitation light polarization is analyzed by taking into account of the parity symmetry of molecular orbitals in the blocking unit of rutile TiO2. This work reports a direct measurement of valence band structure and hole effective mass of rutile TiO2(110), which will deepen our understanding of the electronic structure and charge carrier properties of the model material and provide reference data for future theoretical calculations.