Paper SS+AS+HC+OX-WeA12
Reduction and Oxidation of Transition Metal Oxides: From Tailoring the Surface and Interface Properties to the New Crystalline Phases Formation

Wednesday, October 23, 2019, 6:00 pm, Room A220-221

In the recent years transition metal oxides have attracted tremendous interest, mostly due to the manifold real applications, ranging from (photo)catalysis, through memristive and neuromorphic device development, to energy storage and production. A specific quality which makes them so versatile is the ease by which their electronic and structural properties can be controlled by changing a cation’s reduction state.

In this presentation we will present an overview of the impact that thermal reduction and oxidation have on the surface properties, which enable a precise control over the valence state of prototypical binary and ternary oxide representatives: TiO₂ and SrTiO₃. We will focus on the preparation methods under regular UHV conditions and upon additionally reduced oxygen partial pressure.

Reduction of both crystals results in the formation of oxygen vacancies and therefore d-electrons, which leads to changes in the work function and a corresponding rise in electrical conductivity, which could be tuned over many orders of magnitude [1]. A newly developed SPM-based technique, combining LC-AFM and KPFM, allows both measurements to probe the same area of the reduced TiO₂(110) surface [2], helping understanding of the nanoscale resistive switching . Besides the change in electrical properties, the surface structure evolves towards nonstoichiometric reconstructions [1], due to the increased oxygen deficiency. Surprisingly, not only is oxygen flow possible during UHV annealing of the oxide crystal, but also incongruent cation sublimation can be triggered, as demonstrated for the perovskite oxides like SrTiO₃ [3]. Extremely low oxygen partial pressure (ELOP), achieved by the use of an oxygen-getter, initiates SrTiO₃ crystal decomposition and the formation of stable monocrystalline cubic TiO nanowires with a c(4x4) reconstructed surface [4]. Such bottom-up growth of conductive TiO nanostructures could be an alternative to other costly methods, resulting in the creation of the TiO/SrTiO₃ interface, with a sharp transition between Ti²⁺ and Ti⁴⁺ states, proven by atomically-resolved electron microscopy. This oxide heterostructure provides an interesting metal/insulator junction with a 0.6 eV work function difference [5], opening many new possibilities for (photo)catalysis and aiding in the search for exotic interface states.

[1] Wrana, D. et al. (2018) Applied Surface Science, 432, 46-52.

[2] Rodenbücher, C. et al. (2018) APL Materials, 6(6), 066105.

[3] Rodenbücher, C. et al. (2017) physica status solidi (RRL)–Rapid Research Letters, 11(9), 1700222.

[4] Wrana, D. et al. (2019) Nanoscale, 11(1), 89-97.

[5] Wrana, D. et al. (2019) Beilstein Arch., 201912.