Photoemission electron microscopy of TiO2 anatase films embedded with rutile nanocrystals

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.