In reduced TiO2, electronic transitions originating from the Ti(3+)-induced states in the band gap are known to contribute to the photoabsorption, being in fact responsible for the material's blue color, but the excited states accessed by these transitions have not been characterized in detail. In this work we investigate the excited state electronic structure of the prototypical rutile TiO2(110) surface using two-photon photoemission spectroscopy (2PPE) and density functional theory (DFT) calculations. Using 2PPE, an excited resonant state derived from Ti(3+) species is identified at 2.5 ± 0.2 eV above the Fermi level (EF) on both the reduced and hydroxylated surfaces. DFT calculations reveal that this excited state is closely related to the gap state at ∼1.0 eV below EF, as they both result from the Jahn-Teller induced splitting of the 3d orbitals of Ti(3+) ions in reduced TiO2. Localized excitation of Ti(3+) ions via 3d → 3d transitions from the gap state to this empty resonant state significantly increases the TiO2 photoabsorption and extends the absorbance to the visible region, consistent with the observed enhancement of the visible light induced photocatalytic activity of TiO2 through Ti(3+) self-doping. Our work reveals the physical origin of the Ti(3+) related photoabsorption and visible light photocatalytic activity in prototypical TiO2 and also paves the way for the investigation of the electronic structure and photoabsorption of other metal oxides.

Although it has been widely accepted that the crystal phase, morphology, and facet significantly influence the catalytic and photocatalytic activity of TiO2, establishing the correlation between structure and activity of heterogeneous reactions is very difficult because of the complexity of the structure. Utilizing ultrahigh vacuum (UHV) based temperature-programmed desorption (TPD) and density functional theory (DFT) calculations, we have successfully assessed the photoreactivity of two well characterized rutile surfaces ((011)-(2×1) and (110)-(1×1)) through examining the photocatalyzed oxidation of methanol. The photocatalytic products, such as formaldehyde and methyl formate, are the same on both surfaces under UV illumination. However, the reaction rate on (011)-(2×1) is only 42% of that on (110)-(1×1), which contradicts previous reports in aqueous environments where characterization of TiO2 structure is difficult. The discrepancy probably comes from the differences of the TiO2 structure in these studies. Our DFT calculations reveal that the rate-determining step of methanol dissociation on both surfaces is C–H scission,; however, the barrier of this elementary step on (011)-(2×1) is about 0.2 eV higher than that on (110)-(1×1) because of their distinct surface atomic configurations. The present work not only demonstrates the importance of surface structure in the photoreactivity of TiO2, but also provides an example for building the correlation between structure and activity using surface science techniques and DFT calculations.

We have used two-photon photoemission (2PPE) spectroscopy and first-principles density functional theory calculations to investigate the electronic structure and photoabsorption of the reduced anatase TiO2(101) and rutile TiO2(110) surfaces. 2PPE measurements on anatase (101) show an excited resonance induced by reduced Ti3+ species centered around 2.5 eV above the Fermi level (EF). While this state is similar to that observed on the rutile (110) surface, the intensity of the 2PPE peak is much weaker. The computed oscillator strengths of the transitions from the occupied gap states to the empty states in the conduction band show peaks between 2.0 and 3.0 eV above the conduction band minimum (CBM) on both surfaces, confirming the presence of empty Ti3+ resonances at these energies. Although the crystal field environment of Ti ions is octahedral in both rutile and anatase, Ti3+ ions exhibit distinct d orbital splittings due to different distortions of the TiO6 units. This affects the directions of the transition dipoles from the gap states to the conduction band, explaining the polarization dependence of the 2PPE signal in the two materials. Our results also show that the Ti3+ induced states in the band gap are shallower in anatase than in rutile. The d → d transitions from the occupied gap states to the empty Ti3+ excited states in anatase can occur at energies well below 3 eV, consistent with the observed visible-light photocatalytic activity of Ti3+ self-doped anatase.

Many physical and chemical processes on TiO 2 surface are linked to the excess electrons originated from band gap states. However, the sources (surface and/or subsurface defects) of these states are controversial. We present quantitative ultraviolet photoelectron spectroscopy (UPS) measurements on the band gap states of TiO 2 (110) with constant subsurface defect density and varied surface bridging hydroxyls (O br H) prepared through photocatalyzed splitting of methanol, in combination with density functional theory (DFT) calculations. Our results clearly suggest both surface and subsurface defects contribute to the band gap states, whereas the contribution of subsurface defects corresponds to that of only 1.9% monolayer O br H at the current bulk reduction level. As the surface defect concentration is usually much larger than 1.9% monolayer in real studies and applications, our work demonstrates the importance of surface defects in changing the electronic structure of TiO 2 , which dictates the surface chemistry.

As a cocatalyst, Pt is well-known for accepting photoexcited electrons and lowering the overpotential of hydrogen production in photocatalysis, being responsible for the enhanced photocatalytic efficiency. Despite the above existing knowledge, the adsorption of reactants on the Pt/ photon-absorber (for example, Pt/TiO 2 ) interface, a prerequisite to understand the photocatalytic chemistry, is extremely difficult to investigate mainly because of the complexity of the powdered material and solution environment. Combining ultrahigh vacuum and well-ordered single crystals, we study the photocatalytic chemistry of methanol on Pt-loaded rutile TiO 2 (110) using temperature-programmed desorption (TPD) and ultraviolet photoelectron spectroscopy (UPS). Despite the same photocatalytic chemical products (i.e., formaldehyde and surface hydrogen species) as on Pt-free TiO 2 (110), the subsequent chemistry of surface hydrogen species and the photocatalytic reaction rate are much different. The bridging hydroxyls desorb as water molecules around 500 K on the Pt-free TiO 2 (110) surface, and by contrast, this desorption channel disappears completely and water and molecular hydrogen desorb at much lower temperature (<300 K) after Pt deposition, which can prevent the recombination of hydrogen species with formaldehyde. More importantly, methanol dissociates into methoxy at the Pt/TiO 2 (110) interface, which is crucial in the photocatalytic chemistry of methanol on TiO 2 surfaces because methoxy is a more effective hole scavenger than methanol itself. The photocatalytic chemical reaction rate is increased by nearly 1 order of magnitude after 0.12 monolayer Pt deposition. This work suggests that Pt loading can promote the dissociation of methanol into methoxy and lower the desorption barrier of molecular hydrogen, which may work cooperatively with separating photoexcited charges to enhance the photocatalytic efficiency. Our work implies the importance of the cocatalysts in affecting the surface structure and adsorption of reactants and products and then improving the photoactivity, in addition to the wellknown role in charge separation.