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Surfaces of Energy Functional Metal Oxides

Final Report Summary - ENERGYSURF (Surfaces of Energy Functional Metal Oxides)

The project has resulted in a number of significant discoveries associated with the surfaces of energy functional metal oxides, four of which are described below. One is related to a model photocatalytic interface between liquid water and a TiO2(110) surface. Using a novel approach to investigate the structure of the interface, we discovered that it is ordered with dissociated water (OH) on every other five fold coordinated Ti atom. A comparison with detailed calculations reveals that the origin of the formation of the hydroxyl overlayer involves the mixed adsorption of O2 and H2O on a partially defected surface. The quantitative structural properties derived here provide a basis with which to explore the atomistic properties and hence mechanisms involved in TiO2 photocatalysis. Published in Nature Materials (2017).
The second is associated with supported metal nanoparticles, which form the basis of heterogeneous catalysts. Above a certain nanoparticle size, it is generally assumed that molecules bond in an identical fashion as on a semi-infinite crystal. This assumption has allowed the database on metal single crystals accumulated over the past forty years to be used to model heterogeneous catalysts. Using CO adsorption on supported Pd nanoparticles as a test case, we show that this assumption may be flawed. Near-edge X-ray absorption fine structure measurements, isolated to one nanoparticle, show that CO bonds upright on the nanoparticle top facets as expected from single crystal data. However, the CO lateral registry differs from the single crystal. Our calculations indicate that this is caused by the strain on the nanoparticle, induced by carpet growth across the substrate step-edges. This strain also weakens the CO–metal bond, which will reduce the energy barrier for catalytic reactions, including CO oxidation. Published in Proceedings of the National Academy of Sciences U.S.A. (2015).
A third highlight is the observation of tangled polarons. Polarons, that is lattice distortions associated with the stabilisation of electronic states, are thought to play a pivotal role in the physics and chemistry of metal oxides. They are implicated in phenomena as diverse as high-Tc superconductivity, resistive switching and photocatalysis. Although it is difficult to access polarons in the bulk material, in principle they can be probed and manipulated at the surface in order to test models of their formation and properties. Here this is exploited using scanning tunnelling microscopy to examine polarons on TiO2(110), with oxygen vacancies (Ob-vacs) as the electron source. By manipulating the lateral positions of Ob-vacs using electrical pulses in STM, we show that the polarons follow the Ob-vac and are therefore not directly associated with a particular donor centre in the selvedge. Polaron hopping is not observed below 16 K. By forming Ob-vac dimers, trimers and tetramers it is possible to make the polarons interact in a manner explained by theoretical calculations. Published in Physical Review Letters (2016).
The fourth highlight is of measurements that follow the electron movements after light absorption by a model light harvesting material, TiO2. A state about 2.7 eV above the Fermi level was observed on a hydroxylated TiO2(110) surface. The onset of the band gap state to the OH-enhanced resonance is 3.1 eV corresponds to the threshold for photocatalysis, suggesting that this photoexcitation process plays a role. Time resolved photoemission was successfully performed to measure the lifetime of the electrons in the intermediate state indicating a lifetime less than 20 fs. Time resolved photoemission using higher harmonic generation has been used to follow the electron dynamics associated with trapping and recombination. The results directly evidence band state state assisted recombination, suggesting a means of decreasing the recombination rate. Published in Journal of Physical Chemistry Letters (2015).