Atomic Scale Observation of Chemical and Electronic Properties of Metal-Oxide Surfaces

To meet the future global energy demand - an estimated additional 10 terawatts (TWs) per year in 2050 - a diversification of energy sources and fuels is needed. Solar energy represents a prominent alternative energy source. An important material for these solar-based applications is titanium dioxide (TiO2), thanks to its stability, band alignment and abundance. TiO2 is either used as a scaffold or to create photogenerated electrons and holes to perform catalytic reactions. However, the large bandgap of TiO2 yields device efficiencies that are too low to be economically sound. To make TiO2-based devices competitive, the electronic and chemical properties of TiO2 need to be clearly understood. In this work, using scanning tunneling microscopy (STM) together with spectroscopy techniques (tunneling spectroscopy (STS) and inelastic tunneling spectroscopy (IETS)), we study the electronic and structural properties of pristine TiO2 anatase (101), the most technologically relevant polymorph of TiO2. In particular, STM-IETS was applied for the first time to obtain chemical identification of adsorbed species on the semiconducting TiO2 surface. For each step, density functional theory (DFT)-based calculations were performed to support our findings. Using STM, we showed that the high reactivity of step edges along the [-111] direction stems from oxygen vacancies (VOs). As studied with STS, this non-stoichiometric step edge exhibits a bandgap reduced by 2 eV. Furthermore, a higher amount of adsorbates are present on this step edge, suggesting a higher chemical reactivity. Going one step further, we created a novel surface phase consisting of undercoordinated Ti atoms, increasing the amount of VOs over the whole surface phase. This new surface phase exerts the same behavior as the step edges, reducing bandgap and enhancing reactivity. On the other hand, by exposing the anatase surface to excess oxygen at elevated temperatures, we reduced the overall surface reactivity. This reduction was achieved by formation of an oxygen network, acting as a passivating layer on top of the TiO2 anatase (101) surface. Additionally, the excess oxygen fills vacant positions at the highly-reactive step edges along the [-111] direction, reducing the overall surface reactivity even further. It is important to note that the preparation procedures of both surface phases with enhanced and reduced surface reactivity are cheap and reversible, only modifying standard ultra-high vacuum (UHV) cleaning methods without any additional materials. Improved characterisation of the interaction between water and the TiO2 anatase (101) surface represents another important aspect of this work. Although photocatalytic water-splitting on TiO2 has been used for over 40 years, fundamental insights of the reaction mechanisms are still missing. In this thesis work, we labeled individual H2O and OH molecules on the semiconducting surface by detecting their vibrational modes with STM-IETS. Through clear identification of adsorbed species, we demonstrated that water can thermally dissociate on the TiO2 anatase (101) substrate without an additional light source. Furthermore, for the first time, we could structurally identify formation of a well-ordered water monolayer on TiO2 anatase (101). The work presented here opens new paths towards fundamental understanding of surface reactions on metal oxides, especially water on TiO2 anatase (101), to improve future solar energy conversion devices.


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