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.
Many physical and chemical processes on TiO2 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 TiO2(110) with constant subsurface defect density and varied surface bridging hydroxyls (ObrH) 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 ObrH 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 TiO2, which dictates the surface chemistry.
Inspired by the significance of hydrogen–solid interaction in hydrogen energy and catalysis, adsorption, diffusion, and desorption behaviors of deuterium atoms in rutile TiO2(011) have been ...investigated by temperature-programmed desorption (TPD) and ultraviolet photoelectron spectroscopy (UPS). Upon exposure, a small portion of D atoms adsorb at surface oxygen sites, resulting in the band gap states at 1.35 eV below the Fermi level and desorbing as water at ∼400 K. Most of the D atoms will diffuse into the bulk due to the relatively low activation barrier and the huge capacity of the solid material. These bulk D species desorb as D2/HD between 500 and 800 K. While the desorbing D2O from surface hydroxyls saturates at ∼0.10 monolayer (ML), the yielding D2 is about 96 ML (equivalent coverage) at the largest atomic D exposure of 4.54 langmuir and no saturation trend has been observed in the present work. Detailed analysis indicates the bulk D will diffuse back to the surface and recombine as D2 at elevated temperatures. The differences between the behavior of H(D) in rutile TiO2(110) and TiO2(011) have been discussed by considering the presence of additional bridging oxygen atoms between the in-plane and topmost ones on the latter surface. The striking finding that most surface D atoms diffuse into the bulk of rutile TiO2(011) will not only broaden our understanding of the interaction of H/D with the prototypical metal oxide material but also provide clues to investigate the mechanism of H/D involving reactions over TiO2 catalysts, for example, hydrogen evolution and hydrogenation.
In reduced TiO2, electronic transitions originating from the Ti3+-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 Ti3+ species is identified at 2.5 ± 0.2 eV above the Fermi level (E F) 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 E F, as they both result from the Jahn–Teller induced splitting of the 3d orbitals of Ti3+ ions in reduced TiO2. Localized excitation of Ti3+ 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 Ti3+ self-doping. Our work reveals the physical origin of the Ti3+ 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.
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/TiO2) 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 TiO2(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 TiO2(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 TiO2(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/TiO2(110) interface, which is crucial in the photocatalytic chemistry of methanol on TiO2 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 well-known role in charge separation.
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•Our study provides the most strictly fundamental data of the α-U up to now by in situ ellipsometry.•The electronic structure of α-U has been calculated accurately, which is ...consistent with the ARPES measurements.•Optical constants of α-U were accurately calculated for the first time by combining ellipsometric fitting parameters and band calculation results.•This methodology of studying electronic structure and optical properties, which consists of in situ UHV ellipsometric and ARPES measurements and improved data analysis model, has the potential for wide applications in studying other active metal systems.
Uranium exhibits high chemical activity. The preparation of single crystalline uranium films via molecular beam epitaxy, followed by in situ characterization under ultrahigh vacuum condition, effectively avoids sample contamination caused by the environmental atmosphere. In this work, single crystalline uranium films were grown on the W(110) surface by a deflected electron beam evaporator. The optical constants of the W(110) substrate and the uranium films were obtained through in situ ultrahigh vacuum spectroscopic ellipsometry, while the electronic structure of uranium films was measured by angle-resolved photoemission spectroscopy (ARPES). Notably, the optical constants of uranium film differ from those previously reported measured by the ex situ experiments. The band structure and optical constants of the uranium film were calculated by the ab initio method, and theoretical calculations were compared with the experimental results. The calculated band structure of α-U is consistent with the bulk electronic structure of uranium films measured by ARPES, except for a surface-state-like feature. The parameters of the Drude oscillator derived from the ellipsometric fitting were used to revise the calculated optical constants, which obviously improves the accuracy of the calculations. Our study provides the most strictly fundamental data of the uranium up to now.
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.
Deuterium kinetic isotope effect (KIE) in the photochemistry of methanol on TiO2(110) has been studied to find the rate-determining step (RDS) and understand the reaction mechanism using two-photon ...photoemission spectroscopy. Deuterium substitution of the methyl hydrogen has little effect on the kinetics of this reaction, suggesting that neither the break of the C–H(D) bond nor the transfer of the H(D) atoms to the bridging sites is the RDS in the transformation of methanol into formaldehyde. In contrast, the reaction rate of MeOH is ∼1.3 times that of MeOD, suggesting that the cleavage of O–H(D) is the RDS in the photocatalyzed dissociation of methanol on TiO2(110). The results contradict the common fact that C–H(D) is more difficult to break than O–H(D) based on ground-state energetics, implying the involvement of photogenerated charge carriers in the reaction of the C–H breakage, whereas the cleavage of O–H is likely a thermal reaction. Difference in the activation energy of O–H and O–D dissociation reaction in the methanol/TiO2(110) system has been calculated based on the KIE measurements. Our work is consistent with the fact that methoxy is photocatalytically more reactive than methanol and suggests that the conversion of methanol into methoxy is crucial in the photochemistry of methanol on TiO2(110) and probably other metal oxide semiconductor surfaces.
