Photoemission Electron Microscopy of TiO2 Anatase Films Embedded with Rutile Nanocrystals

Authors


  • The authors were supported by the Department of Energy, Division of Chemical Sciences of the Office of Basic Energy Sciences. Pacific Northwest National Laboratory is operated for the U.S. Department of Energy by Battelle. Some experiments were performed in the Environmental Molecular Sciences Laboratory, a U.S. Department of Energy user facility operated by the office of Biological and Environmental Research. The authors would like to thank NT-MDT for conductive-AFM measurements and A. Scholl and A. Doran for ALS beamline assistance. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Abstract

Photoemission electron microscopy (PEEM) excited by X-ray and UV sources is used to investigate epitaxial anatase thin films with embedded rutile nanocrystals, a model system for the study of heterocatalysis on mixed-phase TiO2. Both excitation sources show distinct contrast between the two TiO2 phases; however, the contrast is reversed. Rutile nanocrystals appear darker than the anatase film in X-ray PEEM images but brighter in UV-PEEM images. We observe that topography-induced contrast is dominant in X-ray PEEM imaging, whereas work function and density-of-state-based contrast, dominates in UV-PEEM. This assertion is confirmed by UPS and conducting AFM data that shows the rutile work function to be 0.2 eV lower and a greater occupied valence band density-of-states in rutile (100) than in anatase (001). Since the boundaries between rutile nanocrystals and the anatase film are clearly resolved, these results indicate that PEEM studies of excited state dynamics and heterocatalysis are possible at chemically intriguing mixed-phase TiO2 interfaces and grain boundaries.

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