The prototypical photocatalyst TiO2 exists in different polymorphs, the most common forms are the anatase- and rutile-crystal structures. Generally, anatase is more active than rutile, but no consensus exists to explain this difference. Here we demonstrate that it is the bulk transport of excitons to the surface that contributes to the difference. Utilizing high –quality epitaxial TiO2 films of the two polymorphs we evaluate the photocatalytic activity as a function of TiO2-film thickness. For anatase the activity increases for films up to ~5 nm thick, while rutile films reach their maximum activity for ~2.5 nm films already. This shows that charge carriers excited deeper in the bulk contribute to surface reactions in anatase than in rutile. Furthermore, we measure surface orientation dependent activity on rutile single crystals. The pronounced orientation-dependent activity can also be correlated to anisotropic bulk charge carrier mobility, suggesting general importance of bulk charge diffusion for explaining photocatalytic anisotropies.
Oxide monolayers supported or intermixed with an oxide support are potential nanocatalysts whose properties are determined by the interplay with the support. For fundamental studies of monolayer oxides on metal oxide supports, well-defined systems are needed, but so far, the synthesis of monolayer oxides with long-range order on single-crystal oxide surfaces is rare. Here, we show by a combination of scanning tunneling microscopy, photoemission spectroscopy, and density functional theory (DFT)-based computational analysis that the rutile TiO2(011) surface supports the formation of an ordered mixed FeTiO3 monolayer. Deposition of iron in a slightly oxidizing atmosphere (10(-8) Torr O2) and annealing to 300 °C results in a well-ordered surface structure with Fe in a 2+ charge state and Ti in a 4+ charge states. Low-energy ion scattering suggests that the cation surface composition is close to half Fe and half Ti. This surface is stable in ultrahigh vacuum to annealing temperatures of 300 °C before the iron is reduced. DFT simulations confirm that a surface structure with coverage of 50% FeO units is stable and forms an ordered structure. Although distinct from known bulk phases of the iron-titanium oxide systems, the FeTiO3 monolayer exhibits some resemblance to the ilmenite structure, which may suggest that a variety of different mixed oxide phases (of systems that exist in a bulk ilmenite phase) may be synthesized in this way on the rutile TiO2(011) substrate.
Mixed-metal oxide monolayer grown at an oxide support is of great potential in applications like heterogeneous catalysis. In this work, the experimentally obtained ordered mixed FeTiO 3 oxide monolayer supported by rutile TiO 2 (011) surface has been carefully studied with density functional theory calculations. The genetic algorithm based optimization scheme has been employed to improve the searching capacity for possible structures, and a well ordered mixed Fe(II)Ti(IV)O 3 monolayer oxide structure much more stable than the one proposed previously has been successfully located. The new surface structure consists of uniformly distributed Ti and Fe cations in the ratio of 2:1.The simulated Scanning Tunneling Microscopy image based on this model is well consistent with the experimental one. Our calculations have shown that the O vacancy formation energy is rather high at the surface. We have also studied the adsorption of O 2 and CO at the exposed Fe sites on the surface as well as their reactions. The adsorption energies of O 2 are generally higher than those of CO. The catalytic cycle of CO oxidation following an Eley-Rideal type mechanism has been located for CO to react with surface adsorbed O 2 and O.
The oxide/oxide interface for monolayer oxides on rutile TiO 2 (011) substrates is investigated for several transition metal (M) oxides, with M= Fe, Ni, Cr, and V. The samples are prepared using ultra-high vacuum sample preparation procedures and are characterized with a combination of photoemission spectroscopy and scanning tunneling microscopy (STM). Furthermore, stable intermixed monolayer oxides are determined by density functional theory (DFT) based simulations and compared to the experimental results. Experimentally we find that for specific oxidation conditions a monolayer intermixed oxide is formed for all M. Although different structures as well as oxides in different oxidation states can be obtained depending on the preparation conditions, one common structure has been identified for all M. This intermixed oxide appears to have a defined composition of MTi 2 O 5 . For very small amounts of M the surface segregates into a pure TiO 2 (011)-2x1 surface and into domains of the MTi 2 O 5 phase, indicating that this intermixed oxide monolayer is a low-energy line phase in a compositional surface phase diagram. The gas-phase oxygen pressure that is required to form the intermixed MTi 2 O 5 surface increases in the order of V
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.