Exploring routes to tailor the physical and chemical properties of oxides via doping: an STM study

Conversion of Alcohols on Stoichiometric and Reduced Rutile TiO2 (110): Point Defects Meet Bifunctionality in Oxide (Photo-)Chemistry

Oxidic (photo-)catalysts have the potential to play an important role to efficiently implement sustainable feedstocks and green energy sources into future energy technologies. They may be used not only for solar energy harvesting, but also for hydrogen production or being essential for the fabrication of fine chemicals. Therefore, it is crucial to develop a detailed understanding of how the atomistic environment of the catalyst can be designed in order to promote distinct reaction pathways to influence the final product distribution of chemical reactions. In this perspective article, we survey the surface (photo-)chemistry of methanol on rutile TiO2 surfaces and hybrid catalysts based thereon. Especially the role of the surface bifunctionality by Lewis acidic and basic sites combined with the strong impact of point defects such as reduced titanium sites (mainly Ti3+ interstitials) shall be illuminated. It is shown how the selective activation of either O–H, C–H or C–O bonds in the methanol molecule can be used to tune not only the overall conversion, but to switch between oxidative and reductive routes in favor of either deoxygenation, partial oxidation or C–C coupling reactions. Especially the latter ones are of particular interest to introduce methanol from green sources such as biomass as a sustainable feedstock into already existing petrochemical technologies.

Graphical Abstract

A fiber scanning tunneling microscope for optical analysis at the nanoscale

A hybrid scanning tunneling/optical near-field microscope is presented, in which an optical fiber tip coated with 100 nm thick Ag/Cr films scans the surface. The tip metallization enables operating the instrument via a current-based distance control and guarantees sub-nanometer spatial resolution in the topographic channel. The fiber tip simultaneously serves as nanoscale light source, given the optical transparency of the metal coating. The emission response of the tip-sample junction is collected with two parabolic mirrors and probed with a far-field detector. To test the capabilities of the new setup, the evolution of the optical signal is monitored when the tip approaches a gold surface. The intensity rise and frequency shift of the emission provide evidence for the development of coupled plasmon modes in the tip-sample cavity. Photon mapping is employed to probe the optical inhomogeneity of Ru(0001) and TiO₂(110) surfaces covered with silver deposits. While the 2D Ag flakes on Ru give rise to a near-field enhancement, the 3D particles on titania locally damp the gap plasmons and lower the emitted intensity. The lateral resolution in the optical channel has been estimated to be ∼1 nm, and optical and topographic signals are well correlated. Our fiber microscope thus appears to be suitable for probing optical surface properties at the nanoscale.